OK... been fiddling again for the last hour or so!
I have reflashed V1.05 again and I think I might know the problem.. my old HP200i died last week (hence the new Plextor) and the DVD+RW that I have been testing was the very last disc it wrote when it gave up. This disc seems totally unrecoverable even though prior to me burning with the Plextor today it was readable.
The second disc I have been testing with was also used with the old HP200i before it died but after a complete erase, this disc seems to be working ok now.
I have now tested combinations of burning and reading in my Pioneer 106D, NEC3520A & the Plextor 712A and at present, it all seems to be working.
The only other thing I tried to do after first flashing to V1.05 was to set the book type to DVD-ROM in Plextools. Then, when I tried to burn with ULead DVD Workshop, it would keep crashing out which could have been a coincidence but I have not had it ever do that before.
So, it looks as if the problems stem from whatever the old burner did to the discs whilst on its last legs. I guess I will just have to put up with not being able to change the book type setting (which I have learned can't be done with DVD+RW on this drive anyway).
Is it common for discs burned with one type of burner to cause problems when they come to be written in others? The HP200i is over 2.5 yrs old now and was one of the first DVD+RW drives....