I've just taken the old 1937 AJS for its first MOT (which it passed!! First time back on the road since 1942!!) and as I got it out I noticed a sizable patch of oil underneath. When I started it up, it spurted oil out of its breather for a few seconds and smoked like a two stroke for a while until it settled down. Is this the dreaded Wet Sumping? Why has it started to happen all of a sudden? How do I stop it doing it?

Technically, wet sumping is a failure to scavenge adequately when running. You're suffering from the oil tank draining into the sump when not in use which is not quite the same thing. The simple answer is to use it daily !

How long had it been standing ?

It's been standing for a good 18 months, perhaps more and I've been going out and kicking it over regularly to keep the battery topped up, so I don't understand why it's suddenly started doing it?

AMC blkes wetsump just as much as any other brand.Because of their pump design they will leak more in certain conditions due to the position of the oil pump position when the return port is open.