Both my bikes are covered with the ETA
http://www.eta.co.uk/ and I have never had a problem with them. Someone tried to steal my bike a few months ago and they sent out someone to cut the lock off and if the bike was unridebale I would have got a lift home as well.
They will insure pretty much any value bike against any kind of theft providing you have the correct grade locks and its left inside your house. I think they also do race cover but im unsure if that applies to off road use as the risk in the bike breaking is a lot higher.