S2 1600 Junior a bit sick

I'm new on here and only recently purchased my car (although it features here in a very thorough build thread by the previous owner). It's a beautiful car and was running great. Then one day a couple of weeks back it started losing power and felt like is was running on 3, then 2, then 1 cylinder. Once cooled a bit it would recover enough to get me a short distance then start failing, with the occasional backfire.
I have just had another look, changed the spark plugs and had another run. It goes fine until up to temp and then the misfiring and dropped cylinders start again. I changed the coil and still no joy. The car is running a 123 distributor and a Bosch Blue coil. I'm thinking ignition but maybe it's fuel? It has a mechanical fuel pump that seems to be pumping fuel correctly.
Any thoughts appreciated.