A small "progress" update, FWIW..
I tried starting the car from cold again earlier and it started fine but had a bit of a rough idle and again wouldn't rev to more than about 2k rpm. Keeping a light throttle and the revs below 2k resulted in varying engine speed / surging within a range of about 200rpm.
I've removed the MAF and ducting to get to the throttle body and sure enough as predicted by clan the butterfly is very stiff - it can be moved by hand but is too tight for the return spring to do its job and the butterfly just stays where it's put.
I've sprayed a load of brake cleaner in there (the closest I can get to carb cleaner); it's shifting the carbon on the surface of the body but not doing a lot to the free up the butterfly. Looks like I'm going to have to remove and strip it; however it looks like quite a lot needs to be removed and I'm currently stuck trying to remove the engine cover for better access, due to the one stripped bolt keeping it in place
Not sure where to go from here as I lack any decent facilities - my first thought was to drill the head off the offending bolt but I've got no power supply to hand and don't have a battery drill
