Thanks for the suggestions guys. Thought I would come back with an update....
fsck - was clean
posde - your suggestions rely on mysqld running which it isn't - but I may need those as the next step anyway.
The failure seems to be on applying the binlog when attempting to recover from a crash. The system was obviously left in this state from the power failing. It seemed to be stuck trying to apply the logs. Tried lots of different command line options to mysqld and running through debugger etc but nothing really worked.
At the last minute last night I thought I might just remove the binlog files so that it can't apply them. This worked to an extent - in that it now allows mysqld to start. It has quite a lot of DB errors now with things being out of sync and missing which I figure is a side effect of removing the files. Went to bed after that as I couldn't keep my eyes open.
Will now try putting the binlog files back to see what happens. If it is still not happy then will follow posde suggestion and re-install the DB's.