Flameport, delighted to say I managed to free the plug, using a bar. Socket doesn't seem damaged, but I won't use it again ! Thanks SO much for your help. :-)
Flameport - I know you didn't mean smash up the kitchen ! I'd be afraid that I'd break some tiles. The plug is quite a fetching shade of grey, so wouldn't look TOO bad if were there for ever. :-)
I've had another look at the socket, and feel you must be right. Will just have to get it replaced ! The annoying thing is that the plug will come halfway out, but no further - GRRR! Many thanks, anyway.
Does anyone know how to free a three pin plug which has stuck in the socket? There are no screws visible on the front of the socket to let me remove it from the wall.