Spoke too soon. It’s still happening.
I closed my laptop with various tabs open, as you do, including the site.
When I opened it this morning the visible tab was my email.
I clicked on to the tab for the shop, and it was back to £0.00 and 0 in the header cart. There were no errors in the console. I’m sure when I last looked at the page before going to sleep it was showing the correct numbers, otherwise I would have noticed!
Opening the off-canvas cart, and then going to another page caused it to come back.
So although what I did last night has sped the server up, it wasn’t the cause of the problems.
If anyone has any ideas I’d be very very grateful. Thanks.