Thanks again for the replies.
Yes, the car is an Audi. I brought it to an Audi dealer who charged me €85 for the diagnostics tests but still couldn't figure what the problem was. Reckoned it was a turbo-related problem. Suggested I didn't drive any further until they had time to investigate further i.e. take the car apart and charge me a fortune. He suggsested I didn't drive it in the meantime.
Brought it to a well recommended non-main dealer who carried out the diagnostics (for free), was equally baffled but rubbished the notion that it was somehow unsafe to drive.
Then brought it to a friend of my wife's family who also has the diagnostic testing facility. The test showed up an error which he said prompted the engine to go into "safe" mode. He was able to temporarily "lift" the fault, thus returning the car to full power. He reckons the problem is a valve which he needs to order.
Meanwhile the car has returned to safe mode. The other night, I tried a racing start from traffic light to see if it would make any difference. I put the car in first gear, held the clutch down and revved hard before relaesing the clutch. Hey presto, the car returned to full power and has been going well since (that's about three days, now).
No idea why or how this should make a difference but it has.