It will be interesting to find out what is behind this gun, who made it? R.A.York known as lock maker from York & Wallin and gun been made within 6 years period in early 90-th. Barrels were from London trade, apparently H&H... Seller calls it as Best SLE, but didn't mention about demi-block or word Best includes this meaning?
On the whole the gun looks like first hand makers product and price 12K pounds for the allmost new SLE gun with London proof marks sounds unbelievable.
http://www.guntrader.co.uk/GunsForSale/detail.php?NewGunID=100704213205660photo
http://s41.radikal.ru/i092/1007/1c/f1e1466fdfc2.jpg