Hi, saw this thread and thought I'd add an input.
The fault with the knock sensors can give sporadic faults for missfires but usally in only longtitudinal engines where the connection to the exhaust is more rigid and direct. It's unlikely in the A2.
I would suggest that the fault with your car is either coil(s) - rememebr, one failed coil can cause missfires on more than one cylinder as the others over work to compensate - could be plugs (when were they last changed? Failing that, only leaves an air leak or poor fuel (FSi NEEDS GOOD 98ron to operate in stratified mode). Air leaks are common, is it a 1.4 or 1.6FSi?
The FSi's suffer with breather system breakages and the oil filler cap can leak too! All give silly faults.
If checking these gives no answer I can happily run through test plans for you for compression/head gaskets (1.4mainly affected) etc. But I reckon unlikely to be anything major!
Hope I can help!