You will usually only get 'face value' resistor readings on isolated components Rob: Rest of the circuit affects the readings.

Fair comment. 3 of the "off"readings are coming off resistors that appear to be set up in parallel and adding the 1/r values gives me the right resistance for that as a circuit.
Leaves me with one now, the circuit diagram would be useful

i gave up and ordered an ebay replacement... which according to Sod's Law means i will definitely have fixed the existing one by the time the replacement arrives or that Marty will announce plans to return to the UK 🙄

Sensible move Rob: Then when the replacement arrives you can compare/probe them 'in action'
(although I still suspect the through-plating)

For the likes of identifying serial comms. pulses etc you can get a DMM/Scope for about £50,
or, and even more 'Cheap and Cheerful', something like the DSO188 will probably suffice:
https://chinese-electronics-products-tested.blogspot.com/p/dso188-matchbox-scope-tested.html

Incidentally Car Manufacturers don't like to make circuit diagrams generally available of course...
that would allow us to repair lots of stuff for a few pounds instead of selling us a £££ module
Obviously that can't be allowed, and we all have to buy new cars - to save the planet...