Hello all again - It looks like the tyres might be my problem after all (thanks Dan3222).
When I first got the car & rebuilt the brakes/suspension, I fitted 2 new tyres to the front but the rears (though non matching) had plenty tread left so I left them on. The ESP programming must be assuming that, as the wheels don't appear to be going at the same speed, it has to 'intervene' as I must be skidding! Precicely what you DON'T want to happen in a corner - it applies the brake to the wheel it thinks is skidding & upsetting the balance......
A similar problem I recall now from a few years ago on my Dad's Passat estate when it had non matching aspect ratio tyres fitted (by mistake). It also exhibited strange handling behaviour. A new set of matching tyres cured it. As the Passat is only FWD though, I hadn't realised the same thing might be happening to the 4WD Audi. Looks like more new boots are required.....
By the way, the jack was purchased fom a company in Mansfield called BH Repairs - as I recall, cost was around £1400 inc. delivery (it weighs about 3/4 of a tonne so hard to deliver! I can give more details if anyone wants. From personal experience, it's one of those tools that once you have it you can't understand how you EVER managed before.....
It has certainly long since 'paid for itself' in the large amount of major work I have carried out on various cars - I can't recommend it enough if you are serious about doing your own repairs & mods.