I've fixed the fuel starvation problem! Ssee earlier blog posts for details on the issue...

I removed the fuel pump, a

Facet Interupt Fuel Pump - Silver Top

and opened it up to have a look inside. I found it works by oscillating a hollow metal bar up and down in a tube. The bar has simple non-return valves on its ends. The hollow metal bar (piston) is a close fit in the tube. When I inspected the piston, I found it was quite scuffed. The inside surface of the tube was also quite scuffed. I reasoned the pump must be worn out. To prove my hypothesis, I fitted an old(er) pump I had - a Facet Solid State Fuel Pump - Fast Road

which I had fitted to the car initially, but had subsequently found it delivered insufficient fuel on when the turbo was at full chat. Incidentally, I remembered that it was "only just" insufficient, and was far better than what I am experiencing now.

So... refitting the "old" pump has transformed the car! It pulls properly in all gears, and the turbo continues to boost for as long as I dared in my short test-drive. I will need to fit the pump properly, or (probably) upgrade it to one which delivers a little more fuel.

You can see photos of the worn-out fuel pump if you select the thumbnail here:

Now, what I must also point out, is that I completely ignored the advice of the manufacturer of the fuel pump. They warn that the pump MUST be pre-filtered. I simply didn't bother fitting a pre-filter, and this must have caused the death of the pump. It probably sucked in some gritty debris from the fuel tank, which prematurely wore the inner bore and the piston... There's a lesson in there somewhere ;-)