Good to see a pic of the Lenton, lookin grand, in original spec too, see it has the optional front dynahub and sturmey archer lighting as well. Is the front hub also dated 1951? (ties in with the rear?). Judging by the lugs, the frame definitely looks 1950/51. Did you get the original Renold Elite chain with the bike?
In my experience the stays as originally provided were always of variable length - you've been unlucky!
Unless the you can swap front to rear, guess your only option is to source a replacement rear V-stay. The rear was always a bit longer than the front, so watch out for that one!