Just to let everyone know that I took apart the fuel tank and carb this afternoon. There were no visible problems - no out-of-alignment gaskets, blocked tubes or visible bits of dirt, no grass in the petrol or anything I could see. Nevertheless, I gave it a good clean out as best I could, and when I put it back together, hey presto! It worked a dream. It now seems to be running nice and smoothly again.

Thanks everyone for your help - your comments and advice gave me the confidence I needed to be able to attack this and I'm very pleased with the result.

Rob