It took nearly an hour to get the old 0-200 going this morning so there was nothing wrong with the battery. After normal priming (accelerator pump), the engine would fire and run for about 5 seconds, apparently on all four. There didn't seem to be any way we could keep it going any longer with extra priming.
Once it did start and run we realised that we had carb icing as the engine would not run below about 1100 rpm. It seemed as if we were getting carb icing even before the engine fired. It took about 15 minutes with intermittent carb heat to get it to run normally. After that, a nice problem free trip to Sandown and back.
I think that, next time, I will preheat the carburettor rather than the mags.