Oh, agreed - the real work was done by the gunsmith, who fabricated a thin bushing to get the barrel to time correctly, and headspacing it with a big tray of bolt heads and a set of go/no go gauges.
Mind you, given the space available there wasn't time to write about setting up the sights (ever tried to do that when the rearsight isn't true? Damn that axis pin - might have been bent as well as peened), the weeks of patient work it took to bed the thing properly, and my Heath Robinson solution to correcting the offset zero.
3MOA - worth bearing in mind factory reject standard for the No.4 was 4MOA. In addition, the bedding on this one could be a lot better and the barrel's an all-but-shot-out 1955 Mk.2 unit. With some proper attention I know I could easily add a minute to the accuracy, but for now it's fun and it's mine.
I don't have the money for an as-new original stock No.4 barrel (~£600+) and I certainly don't have the money to order a custom barrel, bearing in mind the condition of the receiver it's going onto. I'll keep that money for my target rifle!