I imagine it's "the key won't physically turn in the lock" - they do seem to get gummed up after a few years of not being used.
Spraying WD40 straight into the keyhole (eg with the little tube attached to the nozzle) would probably free it in an emergency, but I've heard it said that in the long run this can make things worse when the lubricant congeals & dries.
Just to conclude this saga....Unfortunately an airport car park operator lost my only working key (the one with the remote button). So in theory no-one could get in (in fact the car park company had some device which gave them access (and no crow bar marks!).
Happily my Fiat dealer found that the manual locks (both sides!) were in fact stuck-presumably through complete lack of use over the cars 5 year life. Fixed both for £116. New key with remote now on order-estimated cost over £100.
Thanks for everyones help.
Just to conclude: the new key and "fitting" came to £224!-re-imbursed by carpark operator.