tried the save as... straight away option... so far so good. I think its just one of those things and at least encourages me to save regularly.

One thing that seems to occur before a bad save is file size. The project is about 54Mb in size, and it IIRC it jumps to about 100 Mb in the folder as I try to save it. I dont know if it is trying to save twice, corrupting, or if its something completely different, but I may try and recreate it just to see if it happens more often. Failing that, think its time to mark this as closed and remeber to save often