A better YouTube receiver
I did phone Amazon - or rather I got them to phone me. Which they did almost instantly.
The helper went through the procedure of stopping YouTube and clearing the data.
It seems to have cured the problem, but tonight will be the test as that is when I have previously had problems.
She also told me that there is a new version of the Firestick coming out around the 25th October.
I'll post here when I have given the system some work this evening.