Back with more good news[???}: despite positive comments earlier in this thread, I'd picked up from other fora that Malwarebytes was responsible for these problems with 2004 freezing and hanging. This was reinforced by following MWB's own forum, which relatea several recent updates but concludes by saying that the latest update did NOT deal with the problems experienced with Win10 v2004!
So I just 'quit' the program (quitting 'sticks' even after restarts), resulting in MS Defender/Windows Security taking over all aspects - I'm most impressed that MS have really upped their game in this and now doubt whether a third party Security program is needed (especially at £30 pa!).
The result (after several days of trial): complete cure - so far!
I might see if MWB will agree a refund as only recently renewed subscription!! Some hope.