Well done Roy - that was it. The missing ten miles an hour turned out to be concealed under the pedal: there was a great lump of old insulation/rust/general crud under the mat that was preventing me from getting full throttle (by quite a lot, too).

I would NEVER have thought to check that myself. It's also probably an early warning, or even a late one, that the toe board needs to be cut out and replaced. I have a nasty feeling it's had the lasagne treatment previously.

So now I have nearly all the power I need. I might just try upping the primary jet to 107.5 too: it doesn't cost a lot and it might make a small difference.

I will also re-check the timing: when I did it last, the car wasn't starting at all (bad coil, that was, as I eventually discovered), and I may have been a bit hamfisted. It might also help with starting cold and hot.

Thanks again to everyone for the suggestions and the help.

Do it! Mine has the larger jet and although not a startling improvement in performance it does make a noticeable difference, pulls better.

Well done in finding the cause, Dennis.