By way of an update, and a thanks for the comments and help, here is where I am with it.
It does appear that I had two faulty relays (fuel pump and EFI) which failed within weeks of installation. A local garage checked over the electrics and found no faults.
Since replacing the relays I've had no further problems, but I have one in the glovebox as a spare, just in case.
Regarding the temperature gauge, I replaced the gauge, but to no effect. I replaced the sensor in the cylinder head, which is quite a delicate operation, and the gauge reads fine now, in the top middle sector when warmed up.
Just in case anyone is wondering:
Fuel pump relay failure: No pump noise/click when ignition is switched on and the car won't start.
EFI relay failure: Sometimes runs a bit rough but more likely is a tendency to stall when on tickover.
Both are located behind the glovebox on the bulkhead.
Last edited by Phil+8; 17/09/21 12:20 PM.
