First of all, I want to thank everyone for their input. It's greatly appreciated!
I finally managed to find the issue and get the car to start and run today! It was a stuck check valve in the HP pump as mentioned in one of the forum posts I linked. I think having stood for 4 years, the fuel has gone a bit sticky and needs diluting with fresh diesel or draining from the tank.
To fix it, I removed the starter motor to get better access to the clamps holding the hose to the inlet of the pump. I removed the clamps and hose and inserted a clean punch into the inlet and found the valve to be stuck. I applied a bit of pressure and it came unstuck. I then pushed it a few times as initial movement wasn't very smooth but it soon improved.
I then put everything back together however one of the hose clamps was a weird design I've never come across before. It could be undone with a flat bladed screwdriver but needed a special tool to do it up. I replaced it with a regular fuel hose clamp borrowed from my '73 Triumph Dolomite as I didn't have anything else to hand!
For anyone trying to diagnose a similar issue, I'll leave some helpful links here:
Multimeter Tests for fuel pressure and cam sensor signal
https://www.the75andztclub.co.uk/for...ad.php?t=69308
https://www.the75andztclub.co.uk/for...d.php?t=109615
Additional info on pressure sensor voltage
https://www.mg-rover.org/threads/cdt...1/post-5027500
HP Pump Check Valve
https://www.mg-rover.org/threads/75-...p-pump.329789/