Just to jump in here and hopefully not upset anyone, brand new No4 Barrels are available in the UKicon and Fultons of Bisley are fitting them. They are made by Lothar Walther. The barrels come chambered and profiled. They are .303 bore with .311 groove. Unfortunately they are 4 groove RH twist so may not be acceptable to the purists. The outside diameter is also slightly larger than the original so some relieving of the fore-end may be required. I had one fitted two years ago by the gunsmith who imports them (not Fultons) for £245 including proof. Fultons i gather charge quite a bit more. Armalon barrels fitted are about £350.
I had no choice as but to have a Lothar Walther fitted as I could not find an original barrel cheaper.
My no4 is now shooting reliably 2MOA out to 1000yds.