This came back to bight again today and playing about I found if I deleted all files except the dwg or put the dwg in a separate folder and opened the dwg with audit checked then it opened and saved and I could then get on with work.
Running an audit seemed to fix the problems.
I suspect it was one of the other files that caused the problem.
The frustrating thing was that when Beta testing the AutoCad version I ran several of my old 'Savedwitherrors' dwgs through it and it opened and saved and I could keep working and saving without any problems.
I don't know what invokes the issues - suspect copying between dwgs (MSCAD routine) from paperspace to paperspace is one thing not liked.
But AutoCAD version didn't have an issue as I deliberately put it to the test in Beta testing.
Is there a fix coming for these in the ICad versions please?