Interesting looking flute on ebay. Missing a key that would need to be replaced but still tempting.
https://www.ebay.com/itm/Rare-Hawkes-son-excelsior-class-28-Leicester-London-Wooden-Siccama-Flute/163659409822?hash=item261adc699e:g:hfMAAOSwESZbLRDE&frcectupt=true
That does look interesting. I saw that earlier and got excited, but then I noticed the sounding length of 573 mm, which I thought would make it a high pitched instrument.
I don’t see the sounding length as a very useful measurement since you don’t know what tuning slide extension the flute would be played at. More useful is the spacing from C# to D holes or the measurement Terry McGee advocates which is the C# to E flat hole distance. My experience with several Hawkes and Son eight key flutes is that they played well at A440 with the tuning slide extended 20mm or thereabouts.
You are right Dave! This is going to sound a bit ridiculous, but I went through my collection to measure a few other, hopefully similar, flutes and to my surprise I found that I already have a Hawkes & Son Siccama! That is why I said I saw it earlier. I was thinking of one that I did see earlier, but it was a different one that I actually bought, did some work on, and then forgot about. These things seem to be happening to me more and more recently! 
Anyway, I took some measurements from mine, stripped the keys off it (because they need new springs and pads), blocked the holes and played it in front of a tuner. Its sounding length is 573 mm, just like the one on eBay, and it plays well in tune at A=440 hz, well-balanced across the scale, with a tuning slide extension of 17 mm. So, I think that one you pointed out is very likely to be a good one!