I am interested to know what telemetry receiver(s) you are using then Bert? Also, how is the telemetry useful for "getting a good tune"? (I probably should already know this - pardon my ignorance).

The reason that I abandoned use of telemetry receivers years ago was that I had a run of FrSky telemetry receivers failing. I have a thread documenting these failures (and crashed planes), over on rcgroups. (More than one failed on the bench!).
(A flying buddy also had similar experiences and we both bought up and since use "V" series FrSky receivers. Zero problems experienced with them, and I have not read of one single failure from other flyers.)

Nothing in this regard is irretrievably locked in for me; I could now use another brand; maybe FlySky which to my knowledge, have a good reputation. Also, they produce a 10 channel rx. which would add an extra advantage as I am running out of channels on both my two stroke powered tugs; resorting to sharing one channel for two control surfaces etc. (Y leads).

So, to sum up; what Chris has provided for me will get the project moving forward, which is my primary need.
His assistance has increased my understanding of arduino basics and with some further work on my part in due course, I may be able to get a "full" sketch to verify.
I am completely open to flashing my XIAO board with the full sketch and to order another 10 channel FlySky receiver. (I have one in commission somewhere - cannot remember this morning!)
I have no legal flying field locally, so cannot fly until sometime next year - so I have plenty of time to experiment and modify etc.
The little XIAO board can be flashed (uploaded) with the limited sketch and then flashed again with the full sketch when and if I get a sketch to verify on my system.

Thanks for reading all this, I did not intend to clutter the discussion up with extraneous matters.

Jim.