Just in case any of you are having similar woes, I left the car in with the dealer yesterday, there was a small hole in one of the airlines and the compressor was loose (hence the noise). All fixed now for a relatively small sum of money and appears as good as new.
Big thanks to Emily and all at JJ Reid in Letterkenny for the quick repair + excellent service at short notice. Pity about the VW Jetta courtesy car though, it was truly the most soul destroying car I have ever driven........... it it has served to remind me how good the A8 is though.