Hi All,
Thought I'd provide an update after having Audi check this out as it happened again.
They first asked what sort of fuel I'd been running, perhaps expecting me to say 95 RON, with the implication being (I think) that running on 95 would result in the engine tuning back/misfiring like I experienced when it got hot. (sorry, I'm not mechanically minded at all....)
But I've used nothing but the BP Super Unleaded stuff.
Their conclusion was that they don't know what is wrong, some other customers have reported the issue, and for now they are just monitoring. All I really want to do was get it logged that there had been an issue so if there's a serious problem down the line they can't blame me.
Their advice was to not idle the car with the engine running, and always keep the Start/Stop switched on, so it seems to me that there may be a temperature issue with a completely stationary S3 having it's engine running for 20 mins or so, not something I've had an issue with before.
If you're wondering why I would such a thing, it's my understanding that the aircon doesn't work with the engine off, and to be honest I've been able to idle every previous car for 30 mins plus without one ever overheating, so never thought I was doing anything particularly odd.
They said that the car didn't overheat from a water/oil perspective, but rather the petrol behaviour triggered a sensor when the engine got particularly hot.
No idea if I was fobbed off, but 2,000 miles in with the car, it's running beautifully and I love it, so it doesn't bother me unduly.
Cheers!
Nick