Marc, where are you? I can diag the car for you if you like. For now, check for air leaks everywhere, pour water or whatever over the inlet and check for changes in idle. Check all the resistances on all the sensors for motronic. it's not a complicated system, you should be able to figure out what's going on if you're methodical. And you're correct, nearly all UK cars have no O2 sensor and no cat. We have a different eprom in the ecu to US cars (same ecu).
high idle means something is wrong, most likely air leak. Check the rubber bellow from AFM to TB for splits, and the hoses to the ICV. Make sure the TB idle scew is backed out all of the way, and that there's slack on the throttle cable. check your plugs too, are they sooted?
