Sorry to be slow coming back - no 'puter time over the weekend.
OK, I won’t disagree with what the others have said. However, I have been something of a supporter of these flutes as players’ instruments, but with severe caveats! I have experience of doing several of them up with a view to selling them on. Regardless of whether they have makers’ stamps, some turn out to be well made with good internal tuning and can make decent beginner/intermediate instruments. Others are appallingly badly made, have dreadful internal tuning issues, badly cut embouchures so they won’t “speak” properly, etc. Also, a great many of them were made for old Continental diapason normal - c A=430-5 - rather lower in pitch than we play today. Those ones are of no practical use to today’s players, regardless of their quality. Until one has restored an instrument to playing condition (obviously a gamble!) it is impossible to be certain of pitch, internal tuning, general quality of sound/tone/“voice” etc. - and therefore impossible to put a firm value on it.
We can make a good estimate of likely playing pitch if you can supply the following measurements of your flute, which will give a slightly better idea of potential value/whether it is worth expending any effort on: with the flute assembled and all parts pushed fully together, measure in mm 1) the overall length including the crown and foot caps; 2) the “sounding length” - from the centre of the embouchure (blow) hole to level with the far, open, foot end; 3) the C#-Eb length - from the centre of the top (nearest mouth hole) open tone-hole (for L forefinger) to the centre of the Eb key-cup (the in-most key on the foot-joint, the short one for the R little finger - make it easier to measure by rotating the foot-joint so the key is in line with the open tone-holes).
Regarding the markings, your photo makes clear that they are what I suggested they would be in my previous post - and such workshop markings are pretty generic on these flutes - they can’t help with provenancing it to a specific workshop/maker at all as their use seems to have been common practice in the Markneukirchen and other German/Czech woodwind trade.
You can read more about these kinds of flute here and here.
As for insurance, I’m sorry if this disappoints you regarding a treasured family heirloom, but these flutes are worth very little. They are very common and are not desired by either the Irish trad music community nor the Classical Period Instrument specialists. A fully restored, playing-condition example that is verifiably at modern pitch (A=440Hz) and has decent internal intonation is worth up to about £300-350 in the Irish players market. Examples not at modern pitch are worth very little indeed - under £70 IMO, even if restored, and I would not waste my efforts on doing so. Unrestored ones that from their dimensions are potentially playable at A=440 could be worth £60-100 depending on condition - and that is the category yours falls into. If you sold it on eBay you might be lucky and get c£150 for it as it is in nice, clean condition and has a case. Overhaul and restoration (if there are no cracks or other hidden issues) would probably cost from £80-120, but it is inadvisable to ask a mainstream music-shop woodwind technician to do the job as they tend not to be familiar with these instruments and don’t necessarily do a good job.
I would not bother to take out specialist instrument insurance on it unless restored and being regularly used by a player. As an unused heirloom, in current condition (or even restored), it probably isn’t even worth a “specific item” mention on your household insurance. Just include it on whatever private written list of possessions you keep for insurance/security purposes. As for your request for tips on a valuer local to you - I’m afraid I don’t know of anyone in the Central Southern area of England - but in any case I sincerely don’t think it is worth you bothering… very few modern music shops or antique dealers know anything about these. Hobgoblin in Bristol might have some idea, but I wouldn’t make a special trip… For the kind of value it has, you would not in any case be asked for a written valuation by any insurer.
To support what I’ve been writing, here are some examples currently/recently on eBay from a quick trawl through my eBay “watches” - this for a c3week period - but a typical haul for any such period - there are loads of 'em out there:
This one, http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=130405476736, admittedly in a very gungy state but will likely clean up OK just went for £43.65 - about right!
This one, http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=130416184626 also went for c £47.00.
And this one, http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=250674125498 for c£66.00.
This one, http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=150473805698, again with a short D foot, achieved £85.00 despite its condition - you can see another example of workshop batch marks like yours on the socket end in one of the pictures.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380256990601 just sold for £150 - over the odds, IMO, as like the others it needs overhaul and repair, but I fear an inexperienced buyer may have been misled by the (accurately) quoted stamps to believe it is English.
This one, http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=320568505149, is very visually similar to yours and has been cleaned up and is allegedly playable, though it clearly needs an overhaul and crack repairs: it went for £185. If it plays at modern pitch and has decent tuning, with repairs/overhaul costing another c £100-150, the buyer may have paid a reasonable amount… or they may have wasted their money on a dud (assuming they wanted it to play and not for a pub wall - if the latter, they paid way over!).
This one http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=370416545478 - similar but with a short D foot has been relisted multiple times at £150 to start with no takers.
Here’s another needing overhaul/repair: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=130418367737 - again, a rather high starting price…
And another: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=130405476736.
This one http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=290453250402, maker-branded by a known quality maker has also been relisted several times at too high a starting price to pull any interest.
I don’t have any current links for them, but also for current market context illustration, I’ll mention that there is a Polish eBay seller who regularly offers fully renovated and retuned/revoiced formerly Low Pitch German flutes for about £300-400. I have no idea how well done they are, and am dubious about the whole idea (!!!), but s/he does manage to sell them!
I hope that is helpful and interesting for you. Best of luck with it!