Thanks Sean, You were spot on. Phil I hope this doesn't disappoint!
It turns out that the capture that I posted previously where the O2 sensor remained lean throughout a hard acceleration was the clue to the fix. It did indeed prove that we were looking at a fuelling issue & not a boost problem. I have made some assumptions that I will never make again though . The fuel pressure was only marginally low at idle with & without the vacuum pipe removed. It turns out that Mr VOBD was right on the money with his comments about boost pressure being added to the overall fuel pressure. I did a test on a normally aspirated zetec Mondeo & I was amazed to see that 15 psi added with a Mitty vac to the fuel pressure regulator actually translates exactly to a 15 psi increase in fuel pressure. Nice 1 VOBD (I said one of us was about to learn something hey : )
Now back to the problem Golf. The same pressure applied to the Golf resulted in only a tiny increase in fuel pressure. Next I checked fuel pressure whilst I clamped the fuel return hose so that the fuel pressure would "max out". It barely reached 50 psi. I had the opportunity to do the same test on an Octavia 1.8 Turbo & the fuel pressure reached an amazing 90 psi instantly.
A quick check with an amps clamp on the pump feed wire shows the results of what I am saying
From that single test & capture I think it is fair to say that the pump is fubar. Not only that, it is possible to say what is wrong with it. The segments of the commutator appear to be ok, although there is a slight current dip repeated in one of the segments. The lack of current load as I block the fuel return suggests that the vanes in the pump are partially failed & "free-wheeling" to some extent in the fuel. This was further backed up by the black staining inside the clear pipe that connects to the pump outlet.
As a temporary measure I fitted an in-line fuel pump under the bonnet & did a quick road test & what a difference ;D .
New pump fitted & road tested great. I did a repeat of the capture I posted before & the results speak for themselves...
Next is a classic test that I perform often (Frank Massey highly recommends this test). Studying the relationship between the MAF sensor & Oxygen sensor can reveal allot about the functionality of the closed loop fuel system.
Quite simple in the end I suppose. A mountain out of a mole hill some would say, but no parts were fitted at cost to the customer that weren't required. & above all I learned plenty.
I need to thank Scott & Steve for your input on the phone & everyone else who's suggestions were forthcoming (especially VOBD).
Oh & yes Crasher. Never underestimate the power of the Pico
The above was copied from a post I made on the OBD forum. So some of it isnt relevant to the UKAT thread. I will correct that when I am not so knackered.