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.

The "soggy" throttle response may be due, in part, to retarded ignition timing.

1) Disconnect any/all vacuum lines at the distributor.
2) Set idle timing (as low an RPM as you can manage) at 10* BTDC.
3) At 3500 RPM check that timing is ~35* BTDC.

The water-heated aux air valve (AAV) is an ongoing source of trouble. They either stick closed, or more often, take 10-15 minutes at 80C water temp to close, if at all.

Great information again, thank you. I had the car up on a hoist last night to change the ruptured rear accumulator spheres (apparently DOT4 was once added in place of hydraulic oil) and took a peek at the WUR, (tried) also to eyeball the AAV. In the way of seeing the AAV were the cruise actuator and what’s left of the smog-to-throttle linkage.

The WUR doesn’t look tampered with - encouraging - and the crust of grime blockside of power steering pump suggests nobody’s delved deep for a long while, if at all. So the AAV is probably in a more-or-less original state. I’ve skim read the BenzWorld and Jag Lovers’ threads on AAV dismantling/tweaking, and will be re-visiting them.

Coolant flush number 5 this weekend - have switched to a truck grade product due to amount of fine iron oxides in system - following that will check vacuum lines, and thence timing.