Does it have any real advantage (other than aesthetic) having a longer head over opening the tuning slide a lot?
The antique flute I’ve just bought (details in a few days, or weeks 
) plays at 440hz with the tuning slide opened quite a bit, at the moment (stil getting used to it) almost 1 inch (I’m a sharp blower). Does it matter (other than for internal tuning etc)?
No. And if you had a longer head made to fit the barrel, it would still have a cylindrical bore of the same diameter and the same length to achieve the same pitch. The very slight widening of the bore effected by opening a tuning slide only has very very minor effects, barely discernible if at all, on the intonation behaviour of the flute, and opening it an inch instead of half an inch won’t make a significant difference to that.
As you say you are a sharp blower, the same sort of advice as I and others gave to Chris_Coreline in his recent thread pertains. It will help, may even be necessary (to approach proper intonation) to adjust your blowing technique to get the best out of an antique, and that should flatten you down and let you close the slide up a bit.
OK, so what ya got? Spill!
There might be more chance of a significant leak when you open the slide a long way - but if the response is strong and stable, it’s probably fine.
Garry
Good.
Now, is it possible to flatten the tone and keep the same power (other than turning the blow hole more towards the lips)?
Do you know of great flute players with a strong tone that don’t blow sharp (or that blow even flat!)?
Oh man, another thing to practice…
Here’s the flute anyway, a nice little jem with a difficult emb cut but capable of a lovely strong reedy tone:
https://forums.chiffandfipple.com/t/fs-antique-hy-potter-6-key-cocuswood-flute-sold/74073/1
Right, checked out the old threads. Nice!
As for flattening without losing power by over-covering the embouchure by excessive rolling in, answer is “Yes”. Have a look at the tuning slide settings of almost any good, well known ITM player you can think of and find clips or pics of - they won’t be pulled far out, and they won’t be playing sharp.
Try this. Set the embouchure up on the flute with the front (target) edge in line with the same (“outside”) edge of the C# tone-hole (L1), or even with the emb center on the same centre-line as the tone-holes. Go to playing position (in front of a mirror may be a good plan) and hold the flute so the tone-holes are along the top of the tube, not twisted out or in. Keep your head up so your eye-line is horizontal. Set the embouchure to your lip to your accustomed position. Then “wriggle” the flute down your lip/chin a little - about 1-2mm lower than you are used to, still without turning it in. Now form your lip-embouchure, but arch your top lip more than previously and try to pinch it together at the centre, and let it protrude further than previously. Keep the bottom lip lightly stretched and don’t let it flop out too much onto the embouchure. Now blow, aiming the airstream at the crook of your L elbow or even lower. You’ll take a while to find the focus and sound, of course, but I reckon you’ll notice a difference! You ought thus to play the flute much flatter, better in tune within its own intonation/construction and at least as strongly, if not more, so than before, yet without actually covering more of the embouchure or rolling it in.
You can practice the embouchure technique away from the flute, trying to blow an imaginary crumb off the point of your chin, or putting a finger across the crease of your chin and trying to blow the air as far down the outer side of the finger as you can.
In brief, move it down, don’t roll it in, then blow down and focus better.
FWIW, given your context of hold-changing, this actually ought to fit in quite well with that.
It is quite interesting how much playing style can change the pitch of a flute. I was the previous owner of the instrument Othannen is playing now, and when I was playing it, I had the HJ pulled out about 1/4" to play in tune.
For various reasons, I was not able to play flute for almost two months this fall. When I started up again, I noticed that I played much sharper, with the HJ pulled out about 1/4" to 3/8" more than before. After a month of playing again, everything settled back down to “normal”.
FWIW, all the stuff Jem mentions above worked for me.
Clinton
Thanks, the slide is slowly closing. 
interesting..
I happen to like the sound of most flutes with the slide extended at least 1 cm, but unfortunately I am a fairly flat player and usually I am in tune on most modern flutes at 0.5 cm+/- not more, which is no good when playing in very cold winter venues such as churches, as sometimes I need to play “all the way in”..
Damn old rudall style flutes, I thought my embouchure was good before buying one 
Jem how could you suggest those to beginners!
??? 
'Cos they’re lovely, and if you learn to play on one then you build the proper technique as you go… which has all kinds of advantages! Is exactly why I don’t approve of modern anodyne keyless, characterless cannons!
That said, I went through all that you’re going through over a very long period and wouldn’t claim to have it “cracked”… but then, neither have I when I pick up a modern one - and if it’s a good modern flute, the technique you have developed will pay extra dividends. You started on the wrong thing! And the words I turned pink fall into the “famous last” category - not a good and rarely an accurate thing to think!
People sometimes suggest new heads to improve the tuning on antique flutes. I’ve seen people who had new heads made for ‘improved tuning’. I always wondered, how does that one work,and this seems to confirm that it does not stand up. Of course it’s different if you want a different embouchure cut or if the old one is damaged.
Agreed! I cannot really understand that either. However, even Chris Wilkes seems to subscribe to this to some extent - when I got my new head for my R&R from him he commented amongst either things that he thought it improved the tuning. I didn’t challenge or query that, but I can’t say I notice any significant difference in that respect. The different embouchure cut certainly gives an easier, stronger, tone and is more responsive and flexible in most respects as well as encouraging better focus and air economy. Those things have made me (renew) work on my embouchure (so the stuff I’ve been writing in this and Chris_Coreline’s thread is in part down to recent experience of my own) and that has doubtless had benefits for my intonation - so perhaps the head can influence intonation by how it leads/encourages one to play… but I don’t think the same bore-size and length new cylindrical tube is going to fundamentally alter the physics/acoustics of the flute! Embouchure hole size and depth may have a slight acoustic influence on intonation and pitch, perhaps, but I think very little. If one plays a new head the same way as the original, whilst differences in tone-character may be apparent, I doubt the intonation will change noticeably. You have to play differently for that - and a new/different head may well encourage that.
I’m a sharp blower
therein lies the challenge I think (been the topic of the week around these parts). It’ll come, keep working on it
Dear Lorenzo,
To lengthen the head is a design issue. I design my flutes (of various pitches) according to proportions that I’ve developed. Even Boehm mentions a head that is 1/3 the overall length (?). If I pull the slide out (i.e., longer head), the flute gets flatter, but the octave stretches on the short tube (I mean notes like C# and B). I wrote about Head Length here: http://www.sweetheartflute.com/flute_research.html
In my experience, Head Length is one of the parameters that helps tweak the whole flute as a resonating system: this detail is one of many that helps improve octave registration, compass, dynamic response, speaking rate and tone control. Try picking up a good flute, and when you pull the slide way out, I think you will notice several things out of whack. Head Length is something I start with, and I design around it. It works for me. As to what the makers of old had in mind, or perhaps what accoustically correct position they held, or if they set this length then designed around it, or where your slide belongs, I cannot say.
I find it interesting about the Firth-Pond flutes in the upper body. Here, they are reamed with a straight section for an inch or so before the taper begins. This design effectively makes the head longer. Why not just make the head longer? Guessing, maybe it would look funny to someone. Maybe they wanted to accommodate high trill keys without getting tripped up in the tuning slide.
Walt
there may be more benefit to a new, longer head than the aesthetic aspect of preventing the brass liner from showing. About 7 years ago I acquired a 19th c. flute by Z.T. Purday from a friend of mine (recently deceased, I’m sorry to say). The head was missing, so he had got John Wicks to make a new head, optimised for modern pitch. The new head has an unusually long chimney length, so I guess it’s not just a matter of providing extra length between the embouchure and barrel to allow the slide to be shorter. The whole proportions of the head have to be designed accordingly, I suppose.