Cabriolet - I left it running (at idle) for about 20 minutes yesterday and it was still throwing out white smoke, so I don't think it's just condensation. But then again it doesn't overheat at all. Have changed the head gasket twice, so I'd hope it isn't that. And everyone I speak to says that a cracked block is a very rare thing on a CVH, but it could be I suppose. I'll get a compression tester and see what it comes up with.

Lurch - that sounds like a complete git to find! How did you figure it out??