The final analysis:
The starting problems were cured by Audi replacing both the camshaft and crankshaft Hall sensors. As far as Audi were concerned the car was then fine with all queries answered on the fault codes and their health check.
The erratic running has been cured, in two stages, by MRC as a result of
their heath check. MRC did a pressure test on the turbo plumbing and found two leaks, one in the diverter valve and one in the main pipe from the turbo to the intercooler. (It has a proper name but I've forgotten what it is.) At the time of the health check they replaced the diverter valve with the stock S3 valve in preference to the stock RS2 valve which is pants apparently (I'm quoting), but they did not have the pipe. Audi supplied it, I took the car back today and MRC fitted it. Then, with MRC's 600bhp Doug (his S4, not him) riding shotgun, we went for a little spin.
"We'll drive out of town, over the motorway and onto the dual carriageway. Don't clog it until I say." Oh, dear. The unfortunate chap clearly has to deal with customers with heavy right feet.
The car is brilliant. All the power is back, the turbo comes in perfectly at 3,500rpm and all that will-it-won't-it is gone. Fantastic.
As I said in an earlier bulletin, I should have accepted advice earlier and gone to the specialists first, not second.
£880 to Audi for the Hall sensors (and cambelt and bits and bobs)
£121 to MRC for the health check and S3 diverter valve
£301 to MRC for the main turbo pipe and road test (most of which was the pipe itself)
£1300 to get it right but it felt worth every penny as I drove home. My vigorous but faintly irrational pursuit of dodgy petrol as the culprit has been exposed as advanced level straw clutching.
Those who read my anecdote about fuel consumption and old racing drivers in georgers2's "fuel consumption on rs2", may agree that my role in life may well be to find the most expensive way of doing anything and everything vehicular and then publishing notes on how not to do it.
Thanks for staying with me on this; thanks for all advice and encouragement.
The RS2 - what a car!
When it's running right...

Volcano Black 1995 RS2. Standard.
Gone - but understood to be alive and well and living in Cornwall.