MG
The Classic Marque

Might be worth smelling the oil as some pumps can let fuel into the sump when the diaphragm fails. Also what was the fuel level when it occurred, might be the pick-up in the tank if it starts when the fuel is around half a tank or below.

Thank you, Greybeard. I'll probably treat her to a new pump when we get home. Though the pump seems to be strong - enough to run the engine while pumping fuel out through the overflow at a good rate a few days ago.

Good luck 😁

I suspect you're a victim of ethanol fuel. Depending on what type of mechanical pump your car uses.
If it's an AC Delco type Y pump (very common indeed even today) most variants have the two halves of the pump held together with five screws.
They are susceptible to decay of the diaphragm from ethanol fuel and the diaphragm is also the seal between the upper and lower halves of the pump....

Must admit I assumed it was a thread locker rather than sealant.
I have ordered some Permatex sealant to use.
Took stud out to have a look although have put it back in and torqued it up prior to getting the sealant, hopefully tomorrow.

AFAIK 243 is a threadlocker. It may have incidental sealing effects but I'm guessing that locking the threads is not your objective.
Loctite do hydraulic sealants and thread sealant as well, some of which are heat tolerant.
542 or 572 might be a better bet.

Just an update and to share the solution to my problem.

As all else had failed I thought what I needed was a thin rubber layer around the radiator connection pipe so the capillary tube would lay in that and hence form the seal to the top hose.

Problem, where do I find a suitable rubber strip to wrap around the radiation pipe.

Brainwave. A bicycle inner tube. Quick trip ...

This is quite sudden. We were miles from home when she began spluttering, and a lot of air was going through the filter. I assumed I had a leak in the fuel line somewhere.
Found a friendly garage with a four-post ramp, and he had a good look from tank to pump - nothing obvious. But the rear float chamber overflow was running, which he fixed. (big chunk of crud).
After that, and after the t...

I have some Loctite 243, that’s Lock N Seal isn’t it, blue liquid.
Or is there a better alternative?
Sorry for so many questions!

I remember using Loctite 243 'lock n seal' many years ago, when I had a similar problem with my A35 engine.

I don't think PTFE tape worked.