All sorted !
Having tried repeatedly, and failed, to book the car in with a local Morgan specialist, I took it to a classic car workshop that have done work for me before.
They put a smoke test on the inlet manifold and identified a couple of air leaks, one of which was the TPS.
I had also asked them to look for a fault on the electrical system as the battery would drain overnight. I was fairly certain this had been an ongoing problem.
Anyway they discovered that the supply to the Lambda sensors was the cause of the battery drain. Basically they were wired incorrectly, being supplied when the ignition was OFF and not when it was on. I can't quite get my head around this and am surprised that the car has recently passed the MOT emissions without problem.
I've yet to collect the car, but they've road tested it and it's running fine and the battery has stayed up overnight, which it didn't previously.
I'm now looking forward to a relaxed drive home without having to worry about the car stalling in traffic.