I checked the throttle cable tonight. The bracket can only go on one way, so I'm happy that's sorted. There did seem to be a slight gap between the plastic quadrant the cable clips onto, and the stop nipple on the throttle body. I could get an 020mm feeler gauge in there quite easily, and when I took the cable off, the throttle snapped shut. I wonder could such a small gap make such a big difference to the revs, but maybe so.
Of the two nuts on the cable, the one nearest the throttle is moving but the other is seized with rust and I need to try to free it, to adjust the cable. It's odd, though, because everything went back on just as it came off, so I would have thought the throttle wouldn't have needed adjustment.
I read somewhere about an adjustment screw on the throttle - anyone know anything about that? (while i desperately try to de-rust the thread on the cable housing!)