Sorted now. Two things. My fuel filter was a bit clogged (with bits of red paint which are a mystery to me as to where they have come from) so that's clean now, and I've also replaced my fuel lines.
I'd replaced my fuel lines this time last year, when I started to get the old petroly boot (which I assumed to be float bowl gasket but was in fact split pipe on nozzle). I was perhaps too generous with the line between T piece and LH carb, and that piece of pipe had a curve in it which in the unusual heat in the UK had caused the pipe to kink and restrict the flow. This tallies with the plugs which both looked a bit too lean for my liking, but they are nice and biscuity now.
thanks for help everyone.
