Further information . . .
1. There are no Outlook add-ins
2. I have increased to PageFile size from 4GB to 8GB - I'll see what that does
3. There's something really odd about my .ost files
I think that this goes back to Outlook 365 problems when the whole system was new and Microsoft support played with it for ages to get my mail accounts working. I'm not sure exactly what they did, but they did create some new .ost files.
Anyway, in the Outlook folder I have quite a few .ost files, 8 of which have today's date under "Date modified" so I guess they're all in use - but there are only only 4 mail accounts.

They are as follows:

<my name> - Outlook New.ost 1,700MB
<my email address>.<this PC's name>.ost 3,100MB

<wife's name> - Outlook New.ost 800MB
<wife's email address>.ost 1,700MB

<my gmail address> - Outlook New.ost 130MB
<mygmail address>.ost 160MB

<my hotmail address> - Outlook New.ost 150MB
<myhotmail address>.ost 230MB

This looks to me like an unholy mess ! Any ideas how I can start to sort it all out ?

Thanks !