Finally I’m able to report back on this one. Apart from being long out-of-date, the user’s Safari browser, as Nigel predicted, was corrupted. After numerous problems trying to update /replace her Safari, her Mac guru has installed Firefox for her and suggested she use that in future. All now works as it should. Not a very satisfactory explanation for Safari users – although there seems to be a clear message that if you don’t keep up to date with Safari releases – especially on older Mac hardware – you can get into quite deep trouble!