Thanks for the responses and to answer the questions above......

I have tried a return to factory settings.
I have tried version 818 but not 819 however from Andrew's comments he hasn't done anything to 819 to effect my problem.
I did try another computer and 818 was no better but 814 worked perfectly so this may have started after 814.
The problem is the same with either an existings song or a new song, or any midi style, the main thing is that it only happens where every track is midi. Add just one RT and the problem vanishes.
I don't have any songs downloaded from the internet.

You should also know that Andrew has already got me to try several other things.....
He suggested I run in "Safe Mode" and it made no difference.
He gave me a special test version of BIAB 2021 and it made no difference.
He gave me a special test version of BIAB 2020 and it worked perfectly so again it seems to be a 2021 issue.

Tony