So no idea why - unless air pumps "bed in" after a few days - but my circulation rate has increased slightly.
Was fine before and is still fine, but now fine+. There has been a small but definite difference (1cm higher in outlet chamber - pond level unchanged).

Been doing some tidying up of pipework and wiring. Just the digital ammeters to install now.
These will allow me to monitor by CCTV.
The main air pump will draw a set current when working normally, so any change and I will know that it needs checking out.
Same for the back-up pump (not an air pump). This should only come on if the main pump fails, but again will give me a "normal" current as a benchmark.
The float chamber is already set up to text me if the main pump isn't moving enough water.

So very nearly properly finished.
Will post more pictures when all done.
Video will follow in due course once I learn how to edit, etc.

Many thanks to several enthusiasts/experts on this forum for the inspiration and encouragement.