[EDITED - FALSE SOLUTION - Problem still happening but leavnig this post here anyway, but please see post below]
I think I have solved this. Can’t be 100% sure yet but …
I’ve been moving around the site until it happens, and when it does there are a few error messages in the dev panel of Chrome about slow network (impossible - I have a 300/300Mps fibre connection and a very good server with a great host) and failed to load some Google fonts which presumably caused some other scripts to load (hence the lack of the ajax cart?)
But… what it appeared to be was a badly configured default installation of Apache under Plesk.
Having changed some settings in apache, it now loads the site much faster (still slow compared to a live site, but much faster than it was). If this stops the ‘slow network’ errors, which it should, then I suspect I won’t see this problem again. Fingers crossed.
I’ve also removed any last remaining ioncube files including the licence manager and upgraded to php 7.2 with opcache on.
So far, so good.
Thanks