Been running fine all weekend. Fitted a new fuel filter and popped out one of the idle jets (the one on the opposite side to the engine) and gave it a blow through - it's a 55. Will get a spare for emergency roadside swapouts.

Checked the history that came with the car, and she had a full fuel tank clean by Allon White about 8 years ago, so I imagine it's just a bit of surface rust (hence small particles that are clearing themselves). It all depends if she's been kept with a full tank of fuel or not. It might explain why she plays up with a full tank - slight corrosion on the upper surfaces which gets dislodged when full.

I had similar issues with a 1986 4 seater which had a mechanical fuel pump. I had already checked it was pumping fuel ok by turning the engine over with the fuel line in a jar. Eventually, a friend who was a professional mechanic told me that mechanical fuel pumps can work intermittently when they are on the way out rather than packing up altogether. The flap valves jam or leak. I changed the fuel pump, problem sorted!

SUCCESS!

Finally, all the ducks lined up

Once I got her started this morning (took a while, but she had been sat for three weeks) and idled OK. Half way to breakfast she started playing up again and still playing up when I got home. Whipped the idle jet out, gave her a spray of carb cleaner and she started up perfectly and idled beautifully!

Thanks for all the help and advice, it's been much appreciated. Hopefully in time, all the rubbish in the tank will work its way through, and in the meantime, I can fix it in minutes.

Hopefully in time, all the rubbish in the tank will work its way through, and in the meantime, I can fix it in minutes.