A further note, there’s seemingly either a generous vacuum leak I’m yet to find, or a fault in the thermostatic idle air control (does this happen to the coolant heated type?) as it’s not particularly fussed by screwing the idle adjustment screw into the stop. Fixing the two vacuum leaks spotted on the throttle body takeoffs, has improved the vacuum signal to the economy gauge, but not perceptibly changed pedal response.

I returned the idle air screw to the “as received” position of 1.5 turns out, but won’t be surprised if this needs a tickle once the idle air is otherwise under control.