Setting Windows Update to download automatically but install manually seems to solve the problem.
A similar bug has been around since XP. Once I get home I will investigate further as I know several people who are complaining of PCs running like treacle.
Anyway, problem sorted, and I went for some Stella at the RRH followed by dinner at Fayrouz. Once again Fayrouz was packed, every table being in use. They must be doing something right!