Thanks for your reply Jeffrey - I've been away so haven't been able to reply until now. The problem occurred whenever I performed some task - for instance colour filtering - any change I made wouldn't happen until I clicked on another layer. In fact any task took a while to refresh to the new state. It was like the old days of computer freezing. I suspected that it may have been the graphics card not handling this latest version of photoshop. I have just collected my pc from its annual service the graphics card has now been updated. I've still to test photoshop 2019, but am confident that the problem may have been solved. I will get back to you either way.