What makes the turbo stall when the throttle is closed, I would have thought it would have spun faster, just as a vacuum cleaner revs up when you put your hand over the suction pipe - due to no air movement, so no work done by fan.

I think you answered your own question Sooty, a vacuum cleaner is just that, vacuum, so when there is plenty of it, it WILL speed up because there is nothing for the fan to push against. However, the more air there is, the harder the fan has to push, reaching a point where there is just too much to push and the fan stalls, as in the turbo. The dump valve is there to release this pressure before it gets too high and slows the turbo down, resulting in a better pressure, much quicker when you next put your foot down because the fan is still running at a decent speed. Hope this explains it better?