Your breakdown symptoms point to fuel starvation to me. What about the fuel tank not venting? if and when it breaks down try undoing the filler cap. If there is an inrush of air and the car now start, the filler cap is not venting. The car will run until enough negative pressure is developed in the tank to stop the pump from sucking.
In 55 years motoring I have never had a coil fail - just saying.
As for the Distributor Doctor, he doesn't supply parts for the awful Delco dissy used on the 1850.
It's funny that and probably a coincidence but prior to breaking down I had just put £20 of petrol in.I will check the filler cap tomorrow I presume it should have a small hole in the cap?
Incidentally I have looked on "Autosparks" web site and they say a replacement coil has to be compatible with electronic ignition.Now I know that the coil was replaced by a previous owner because originally it was fitted with a ballast resister coil and now it isn't.
The suggestions on here will be checked out regarding the fuel starvation etc, but if that seems O.K I will replace the coil, and if that doesn't work I will replace the electronic ignition module as they are reasonably priced now.