I also have a Chieftain Thunderbird C and it is much more mellow than the D. Very nice whistle indeed. Forgot to mention it in my post.
I have both ends of high C and enjoy them all, an Oak C that is in very good tune and on the clear, crisp, (not airy) sound, also, Sindt C that’s a regularly played whistle when with the singers in our group. Additionally, the Oz high C in wood which I prefer because I prefer woody tone to all others.
I don’t often play a C whistle - I have a Burke composite C that I like OK, and still have a couple of Sweetone C’s that work quite well. The biggest mistake of my whistle life was trading a Copeland silver C for a folk harp years ago. It had the sweetest sound imaginable. From the discussion here I’m tempted to invest in a Syn C/D set, but really have enough whistles…I think
About Adrian’s comment about the similarity of Ds and Cs by the same maker, I agree with Gumby that whistles by the same maker, even in neighboring keys, often play rather differently.
I had a Sindt set in A/Bb/B (one head three bodies) and even though it was the same head, each whistle had different characteristics. The Bb was the best player. I’m guessing that the head was originally designed around a Bb body.
About FiddlerWill’s wide-bore wooden whistle and Fleximart’s all-alloy whistles, for whatever reason every wooden High whistle and every all-alloy Overton-style High whistle I’ve tried over the years has played like a whistle with a bore that’s too wide. Whether it’s the bore or something about the head design, they’ve played like big-bore High whistles, which have playing characteristics opposite of what I’m after.
Interesting you should mention that – I came across an Overton-style whistle the other day that had a more narrow bore and it played quite nicely. Found it by accident at the Rudolstadt “world music festival”. I originally went there to see Rob Gandara from Carbony and try some of his whistles. But just around the corner was another music shop with a booth and they had a few whistles that looked like Goldies/Thunderbirds. Unfortunately they had no D left, only lower keys and an E. I tried the E and it was brilliant. The seller couldn’t tell me who made them however – I suspected “Made in China”. The bore was much more narrow than on a Thunderbird. But the price-tag was hefty so I didn’t get one. I don’t pay 150€ for an alloy whistle from an unknown maker. I might have reconsidered if they still had a D and I might have been able to haggle. But so I left with an awesome little Generation D (red head, brass) that plays almost like my vintage one and very differently from my two other modern day Generations (the “paisley” model and the nickel) in D. It’s just lovely. Had to do the “throw inside C-tube to remove head”-trick though to get it in tune.
I too like my Tony Dixon aluminium tunable C. Thicker walled, full tone. Hope this helps. 
Loving all your replies, thank you all. I am pretty new so I don’t want to pay huge money (Sindt is out of my league at this stage) I am hearing Chieftan a bit here. What about Killarney? I will also look to buy a ‘G’ and an ‘A’. Any info is greatly appreciated on any of the high whistles! 
If you mean high G, the Generation is pretty good, as is their high F. If talking low/mezzo, then my Tony Dixon Trad A & G are nice but not too expensive, but Shearwater is a good whistle to look at also.
What about Killarney?
Killarney is fine, not cheap at €90 but well made and sturdy. When I first received mine (when they were first out) I was slightly disappointed by the difference in voicing to the Killarneys I already had. The C isn’t quite as nimble and light to the touch as I would perhaps like but it does the job and I have come to live with it (there were no takers when I put it up for sale). For ease of playing and effortlessness I would stick to my C Generations.
To me, the C is an odd one. As much as I like the Killarney D, I don’t like the C as much. The Dixon Trad D I like a whole lot but I don’t care for the Trad C. The same goes for Generations. I have no idea why. But… I do like the Susato Oriole C more than the other Susatos. And unlike a lot of other people, I prefer the Sweetone C over the D, but prefer the Clarke Original D over the C. I guess the point is that whistles vary quite a bit even if they are from the same manufacturer, and on top of this, there are personal preferences thrown in. Some people don’t like the seam on the back of the Sweetone; it never bothered me. Rather confusing, but one never knows unless he/she actually takes it for a spin.
Thanks! I love this forum, very helpful to newbies like me!
Brass wide-bore Burke.