Delete render files within FCPX.
And something weird happened but I think I had been using Transform in conjunction with Ken Burns and may well have messed things up prior to the crash and then trashed prefs and render files and put two and two together and made three. Then I stupidly put it down to trashing the render files, which, with hindsight, was a bit dumb.
I think the problem was there already or was caused by the crash itself. With hindsight the latter is also more likely.
Thanks everyone.
All this learning gets a bit tiring. Off to see Gravity in 3D!
http://www.jamesewart.co.uk