Well, got back to it today.

There are no air leaks.

VAC figures to awnser above quiry..

With and without throttle body electrical loom connected

With Without
idle 17" 18"
1500 18" 20"
3000 8" 19"

Now, the poor "With" figures are down to the fact that with the loom connected the injectors fire irrgularly causing it to run masively lean. I can not really see any evidence of mechanical/air leak/blocked exhaust issues based on those vac readings...

With the loom disconnected (I am guessing forcing a form of limp mode) everything runs much better (though the higher end performance suffers, presumably due to the fact that the ECU is no longer receiving TPS data)

I have checked all the terminals on the throttle body using PICO-scope and (according to Auto-data) everthing is working as it should.

Except that the car as a whole is not.

The owner has already replaced the throttle body on the advise of vauxhall (Not unreasonably given their poor reputation)

Replacing the ECU seems a rather expensive way of testing it!