504 Gateway Time-out during POST

[David]

Just an update ...

Whilst I feel it may be too early to say this is resolved, I have made a lot of
progress.

Tidying up the order of my <location> sections in nginx.conf appears to have
made the PHP execute as quickly as it should.

I'm going to give this a couple more days and will update in more detail should
things stay as good as they are now.

Thanks again all for your help.