Just got back from MRC Tuning in Banbury. What a difference!
Got there about 9:30 this morning.
By 9:35 he has established that one of my diverter valves has split and Vag Com stated that an EGT Sensor was faulty.
By 9:40 the Diverter Valve had been replaced, the system was pressure tested which showed no signs of leaking. MAF meter also fine.
With Vag Com then running, we took the car to the by-pass and did 4 3rd gear runs to 6500rpm.
This confirmed the EGT Sensor problem as the Exhaust Gas Temperature was jumping from 920 - 1350.
He explained that the intermittent false readings from the EGT Sensor were causing the car to run limited boost and this was confirmed by Vag - Com. New EGT sensor £160. Ouch!
Back to the workshop, 25 minutes to fit the new EGT Sensor even though Audi claim it takes 3.5 hours lol must include all their tea breaks
Back out with Vag -Com for another couple of 3rd gear runs.
Vag - Com confirmed all was now good with all readings and measurements as they should be.
So, my wallet is now £340 lighter, but the car boosts like it should now, my diverter valve is not making stupid noises at me and I have the peace of mind that all is ok.
Going to have Cambelt, Auxilary belt, Waterpump and all tensioners changed and full service for £695 all in. Can't go wrong, they really know what they are doing.
I can see a remap on the horizon too lol
Thanks again to everyone who recommended MRC to me, well worth the 180 mile round trip.