Problem appears to have solved itself.
Broadband was acting up yesterday, plus I bought a new microfilter. I uploaded last night when everything was working fine and it worked ok, no problems.
Channel 4 asked for another version, so I uploaded a bigger version no problems this morning.
I think, in retrospect, that my iBook could not cope with Broadband cutting out all the time. So, it crashed to compliment and add to all my problems!
Who knows! But thanks anyway for all your thoughts. Appreciated, as always.
Frances