Andrew, I suppose this is you? Thanks for the comparison pictures. That would then be a sloppy stamped '17, but not a 27. Which would fit to the rifles serial number.
I also have no idea why they would use an Eddystone action for such a rifle, since as said the Commonwealth for snipers used only the Winchester manufactured rifles. It could either be an inter-war trial where they just used what was at hand. But it can also be that this rifle was NOT a UKtrials rifle, but in fact an US trials. I've also seen Frankford Arsenal made sniper rifles, which used a P.14 action (and this particular rifle had quite an odd design with gearings to adjust the scope). Would the UK mark any of their trials rifle with a certain marking?
Rob, the picture of the P.14 with the A5 telescope also shows that the scope is quite HIGH to what it needs to be. I believe it can be lower with modified scope rings. But again, this is just an idea. I'll place a scope backwards on the bases, to see if everything would clear. And measure the distance between the scope bases, to see if it would fit to the 6" spacing of the A5.