Quick update:
So I carefully cleaned the oxidised area on the ECU with electrical contact cleaner and a soft, clean brush and reinstalled it. I also replaced the seals on the regulator, although the old ones looked fine. Unfortunately no joy, it still won't start
I'm going to repeat FrenchMike's voltage tests just to make sure I'm looking in the right area. I'm also going to check the valve on the inlet to the HP pump as suggested here:
https://www.mg-rover.org/threads/75-...p-pump.329789/
If that doesn't solve it, I think I'm going to replace the regulator (assuming the voltage tests still indicate a HP fuel issue). I don't like replacing parts without being certain, but I'm running out of options. I could try and locate a diagnostic computer but I don't know what information it'll be able to give me beyond the voltage tests.
I read on another post somewhere that it is possible to start the car with the HP sensor disconnected. I did this to try and eliminate it as a suspect and the car still wouldn't start, so I think the sensor is OK.
It's seems unlikely to me that the HP pump would catastrophically fail in such a way where the car ran fine one day, then wouldn't start the next.
It's possible that this is still an injector leak off issue as I never managed to confirm whether my results were normal or not, but they didn't seem that bad so I'm not convinced.