Just to report back on this, problem quite possibly sorted now - took it to a mechanic to get a new fuel filter and O2 sensor installed (would have done this myself ages ago, but I don’t have the right size spanner for the sensor, and no way of going under the car to get at the filter) and the difference is instantly noticeable. Don’t know what’s responsible for the change, the sensor or the new filter, but the car now feels much more powerful and responsive at low revs - really felt like I was driving home a faster car than I’d gone there in! Engine has started perfectly too so far, though time will tell if the occasional low-idle-immediately-after-startup problem is gone for good.
If not, I’m now armed with a multimeter, so will carry out some more tests on isc valve, water thermosensor etc. Also have new gaskets for isc valve, idle air valve and throttle body, so will take these apart if necessary.
Fingers crossed it’s fixed though - thanks for all the advice, and particularly to lrjohn for the fuel filter suggestion! I really hadn’t seen that mentioned anywhere else, or in the Grainger handbook where problems with the engine management system are discussed.