Hi Tuxmaster and Happyrock,
Thanks for your responses. Short answer is that Google still goes to Google, not on her machine at the moment so can't check the address that it goes to, but on mine it is http://www.google.es...:en-GB:official
and I'm pretty sure that it is the same on hers. Also, yes we were setting her Google preferences to 'ingles' every time we switched on.
However, I think I now know what caused the problem and how it did it! There was a recent Firefox update, that I had forgotten about and this appears to have changed one of the settings. I was devilling around in Firefox Tools when I found the Options, Privacy tab and on there I found that 'Accept cookies from sites' was ticked as was 'Accept third-party cookies' and this was set to 'Keep until I close Firefox' so I checked my PC and Laptop and these were both set to 'Keep until they expire' and I have had no problems on these machines.
Changing the cookies setting on her machine to 'Keep until they expire' has solved the problem, at least for now. I know I didn't change anything like this before and my wife certainly didn't as she rarely goes deeper that her e-mails, except to change Google Preferences, which she is now a dab hand at! So I suspect either the Firefox update or her Avast AV.
Any further comments or observations would be welcome...