Barry Forster

OK Guys, it's been a long time but finally the A5 has been sorted and I've got to the bottom of what happened and how.
The original crankshaft pulley failed - a flaw in the casting meant that the boss was still bolted to the crankshaft, but the pulley itself sheared away - which enabled the car to be driven.
However, the replacement pully coming off wasn't a design flaw, it was a case of the sprocket being incorrectly fitted and not aligned with the crankshaft - there are 'mating marks' on the pulley and crankshaft that, if not aligned correctly, will result in the pulley coming off again. But this time, it did serious damage. The head had to be removed, reground, and 8 new valves fitted, plus all the necessary gaskets etc. 
So as a result, the A5 is now purring very nicely.
The mechanic effectively accepted responsibility for the second failure and associated works, without any pressure from me I must add, and asked whether I would be willing to pay for the parts and no labour costs. That came to about £1300:00, which, in the circumstances, wasn't unreasonable.
Thank you to all who offered help and advice.
Best regards, 
Barry.