Many thanks to bill c. and Jerry. It is a high end car and only a couple of years old - but I first noticed this when it was a few months old but was told modern cars cruise along even in second gear without much need of the accelerator - something to do with ease for traffic jam driving. It only does the main *runaway* thing when the engine is cold and just warming up for the first 10 minutes of driving or so and only rarely, say about once a month, for some obscure reason (oh, perhaps connected with empty or newly-full tank, so I'll check scenario now I have thought about it).
Problem with taking it to a garage about this is that it doesn't do it regularly enough for anyone to tackle the matter specifically. But thanks again and for the advice to see if it's been discussed on any manufacturer's fora.
(@stud, my broomstick is working just fine so be careful I don't fly in through your window to beat you with it. )