Sorted the bugger! Some may remember it took an age to die down to idle. Whilst checking the vacuum tubes for bungage, as suggested, I squashed one of them trying to get it off and noticed that when I blipped the engine, as I was doing constantly trying to find out which one caused the over-revving, the problem went away completely.
Following the tube round I found a ufo-shaped thing on the other side of the carb with a mechanism holding an adjustable screw which contacted a plunger built into the middle of said thing. When the throttle was opened the screw moved away from the plunger and the plunger moved outwards. When the throttle was released the screw hit the plunger and sloooooooooowly went back in again, corresponding to the falling engine speed. A quick adjust of the screw and all is well again.
Happy Si