Right
I took it all apart and discovered that yes it was the freewheel and it's simply kaput. It freewheels for a revolution or so, then catches and sticks. I put as much oil into it as possible but it made no difference. Put the bike back together and it's just as bad as ever - clanking badly when pedaling, and the freewheel is sporadic. Suddenly the pedals are forced round when you're supposed to be freewheeling. Not good. Not safe!
So i phoned Mayhem. They're really friendly and helpful. They're going to pick it up from my work next week and return me a glorious fixed (or new?) A-Bike "within a day or two". So I can't grumble.