Further update. Well after poking and prodding must have found the drain hole, which with my ancient eyes nearly 80 year young, still cannot see. However rinsing several times with petrol the drain hole now flows! Carb back on apart from the lid and have ordered a new needle valve "just in case" the existing valve is worn and await arrival of this and a new coil from Uncle Roy. Then HT check as Ken suggests. Fit new points and condenser as none have been changed since, and probably some time before I acquired 2CV during December 2017, and ""Sorned until passed its MOT April 2nd this year.

The bit about re-starting easily when it's cooled down sounds quite like a faulty coil to me. That's the kind of symptom I've seen cured by replacing the coil.

I'm guessing the coil is not getting to hot when driving as it is getting airflow to cool it. It is possible, that the dwell is a little high. It may be a good idea to check the alternator is not putting out to much voltage.

Don't change the condenser unless you know it is faulty. Old ones ate MUCH more reliable than the crap ones that are available at the moment. Also, in the circumstances, I wouldn't change anything until you have done what Ken suggested and substitute the coil as soon as it dies, while its still hot. 9 times out of 10, from the symptoms you've described, a good coil will cure it.

Thanks everyone for advice, regarding condenser will leave alone though I do have several many years old, very old, stock of them.

It's not a problem of lack of airflow: a faulty coil gets hot internally, and you probably wouldn't feel it because it has - I think - a jacket of oil coolant.

A faulty coil is a faulty coil, and it works for a bit, cuts out, then has to cool down again before it will work. I've had one just like that.

I think even if you installed a refrigerator around it you'd have the same problem.

So what you need to do is confirm or eliminate the coil, by replacing it with a known good one immediately after the engine has cut out. You can strap two coils together with cable ties temporarily to let you do this quickly.

OP indicated it didn't cut out whilst driving, but would cut out if left to idle for 15 minutes. The difference could be the build up of heat under the bonnet.

Mine used to be fine driving it. Stop and go into a shop, come out and it wouldn't start. A new coil fixed it.

A replacement faulty coil can confirm the issue as long as that coil is cold.

However, finding the cause of the overheating coil is important, or your new coil may go the same way. It could be poor quality, or old age. Over voltage from the alternator, or to much dwell are also causes, as is leaving the ignition on with the engine stopped. These can be checked for and avoided.