Made a daytrip to the AOMCC ToD rally near Gloucester. Had a good ride out on my 1938 Ariel 500 but it ran terribly on the way back. More investigation required.
UPDATE - took the carb and fuel pipe apart yesterday. Carb was OK although there was a bit of debris in the float bowl. The fuel filter was full of muck and I think it was restricting the fuel flow such that there wasn't enough flow when I got on the main jet. New filter fitted and all cleaned. Try it out at the weekend.