Also realised that 7th May is a Monday, when Museums in Prague (and I guess most of Europe) are closed, which doesn't make things any better...

many places in Prague close on a Monday especially the publicly owned museums but most of the main tourist attractions will still be open the same applies to the 8th, so i would not worry about it.Monday would probably best for touring the Jewish area as these places will be open. I think the only difference you will notice on the holiday is that traffic is far lighter than normal, the public tansport will probably be running a sunday service which is still terrific.
Pibs ,cafes and resturants will all be open as usual, offices and public buildings closed but for the avwerage tourist its buisness as usual.

I wouldn't change plans. Agree with unclegus that it doesn't make a big difference for the average tourist if it's a holiday or not. Of course, if you plan to visit something specific then check online in advance if it is open.

The drive Prague - Cesky Krumlov - Salzburg is not particularly beautiful. The roads are very busy and the traffic is thick.

Thank you both, I will stick to my plans as you advise; except that I should now explore doing something else on the day I was going to drive via Cesky Krumlov.

Any suggestions on other places to explore enroute to Salzburg?

Thanks again

To clear up any confusion my post may have caused: Cesky Krumlov is beautiful and well worth to spend some hours there! Only the *drive* is not particularly beautiful. I doubt you'll find a place that's more beautiful than C. K. between Prague and Salzburg.