Well we can finally close this once and for all now.
We fitted that new valve to the car last night and the difference was instantly noticeable. It pulled better when accelerating. This was confirmed today when the car was on the rolling road again. In standard form it made 173.6bhp, Peugeot claim standard to be 175bhp so it's near as dammit.
It was remapped this afternoon and is now producing 205bhp so the car absolutely flies!
The only thing they commented on when it was on the rolling road was that the turbo intake pipe was collapsing at full boost. Apparently it's quite common as the cars get older and the rubber quality deteriorates. There's some sort of sensor in the pipe so it's about £48 unfortunately, but in the grand scale of things it's not much!
So, to summarise, this car has had the timing chain and tensioners all changed, the high pressure fuel pump, the catalytic convertor, it's been decoked due to excessive carbon build up, turbo valves replaced, and now the turbo inlet pipe has been changed. All of this work has been required, not done in preventative maintenance, and it's all been needed at 65k miles!
I'm going to Peugeot in Glasgow tomorrow for an MOT so I'll be having words with the master tech and workshop foreman who told us "you don't know what you're talking about, 140bhp is normal on these cars" as I'm now armed with a rolling road print out showing it making near as dammit 175bhp.
It really is disappointing when you need to go outside the dealer network, do over 1000 miles travelling to independent specialists, all of which was just to get a standard production car running properly.
A big thank you goes to BL Auto's for doing the timing chain for us, and another thanks to Jamie @ SP Tuning in Hinckley who appears to be the only person in the UK who has the tooling to do the walnut blasting on the PSA engined cars.