Well, I did a bit more testing on this thing today and I am happy to concur with the afm diagnosis. I couldn't get to 4V at any point. So, a quick call to the dealer for a starting point.....1200 quid !!

Tried the usual suspects for aftermarket and no go. I thought about Gwyn's suggestion of trying to clean it but the car drove in and I wanted it to drive out so couldn't take any chances.

So that's where the story ends for now, the customer is trying ebay & breakers for a replacement.

Back to the hose pipe analogy on pressure and flow, if the pressure remains constant when you increase demand by removing your thumb (or opening throttle) there is sufficient flow - there is no hidden science here. In an automotive situation, it's not always possible to create maximum demand whilst measuring pressure but that's a different problem. My Primera was gasping like a good 'un whilst the pressure stayed spot on spec so there was absolutely no need to check further.

Thanks all for your input,

Dick