Done it! This laptop has an annoying CapsLock - very sensitive & easily turned accidentally but doesn't have an indicator to show its on.
It's a problem I know about and usually check it (which I did today, just in case). Looks like it double bluffed me!
Fortunately my husband showed me how to check passwords used in preferences and there it was! Can't replace the laptop yet as it's ok in most other respects and he's just replaced his but perhaps I should remove that CapsLock key.
Thanks for your help - apologies for the unnecessary bother!