I read this rather detailed thread with
interest and thought only fair I add to it.
Firstly, don't understand why so much time was spent going through what ifs, buts and general crystal balling. With cars and engines the first thing to do is get your hands dirty and find out the exact problem.
Secondly, as with all mass produced goods regardless of manufacturer reputation (and the ads telling you how good they are) there is always going to be a % rate of failure against the norm and SOL sounds like you got the 0.5% car which suffered an abnormal failure.
In my experience as a car enthusiast the general rule is don't trust a garage UNTIL you know they do good work to a high standard. In the past I have had cars come back with bolts lying around the engine bay, cable dangling out the bottom and on one occassion a mechanic telling me they didn't refit a part as its an optional extra.
My opinion on services is that they consist of 75% of things anyone not afraid of lifting the bonnet can do. For Example, check condition of windscreen wipers & condition/operation of lights, c'mon please.
SOL - where you said the car started heating up to 90 degrees after the service? did you notice any unplugged connectors in the engine bay or hanging from below?
Again in my experience with cars;
1 - I drove a mk1 Golf for 2 weeks without a radiator cap because I had lost it after forgetting to screw it back on. I had to wait 2 weeks for the new part to arrive which meant stopping every 20 mins or so to top up the emptying cooling system and I was in effect cooking it each day (London driving). No Head gasket failure.
2 - Same golf oil pump failure, driven 3-4 times for a couple of miles, with lights flashing and buzzers going off. No damage.
3 - Clio, radiator removed awaiting part - driven multiple short journeys (1-2 miles) with no cooling system. No failure of anything.
I admit these are not comparable to todays 'modern' engines which require £100 oils and decide to stop working because you drive over a pothole. I wouldn't recommend any kamikazee practises on anything fancy. I would also advise against products in a can which promise to 'fix' a head gasket. There are no substitutes for a real part in a can/tube.
IMO Tesco (and the like) is for buying spuds and bread not for warranties, and as such resellers of policies underwritten these elsewhere, they tighten up the t&c's to make a quick profit. Again IMO car (and most other) warranties aren't worth the paper they are written on once you have read the t&c's. Better to put the money in the bank and take a chance.
SOL I hope you get your Audi sorted, if not get yourself an older car, a haynes manual, a toolkit and you'll start to question what the purpose of mechanics and garages actually is for!
Thanks,
Mark.