Beg pardon guys now resolved as reported elsewhere:
Just for anyone's interest I took the throttle assembly off reasoning I would get nowhere if I didn't. Much tipex all round to mark the position although there seem to be some radial marking on teh arm whcih I hadn't seen last night..
Then the blessed plug with the allen head didn't want to leave but eventually I got that out.
But I seemed to be looking at the back of an arm so squirting WD really just splattered it all over the place.
Plan B dribble plus gas oil down the shaft from the top and fill up the recess the rod on the diaphragm goes into with penetrating oil. Still no joy.
Then I thought well I have to move the pesky arm and I got a small drift and hammer and gave it a bop. It did move and then I had to force it back using the diaphragm. As you can guess it was lots of small steps, millimetres only and eventually out popped the diaphragm.
At this stage I could begin to really work the rod back and forth and it's pretty well freed up but I'm not totally happy that it's not still got dirt in there. However, I can't rotate it (nor get it out) so I'm stuck with moving it a very small amount to ease it off.
I have some liquid grease in a aerosol can which I think I'll use as the final lubricant but I'm not sure how much more waggling I should do.
Will it work though? I see no reason why not but since when has that mattered?
There's a moral here. If you lose all boost don't despair as it might just be seized up bits so don't rush to get a new pump fitted at huge coost.
Also there is a breather in the boost pump and that is probably what let the damp in that caused the problem so extend that breather I know I shall. It's the dippy little rubber cap with a spout near where the pipe arrives from the turbo. I bet loads of people just chuck that away!
I'd like to record my specific thanks to Andy at Allipsort who was really very helpful sharing his expertise freely. Top man and while I'm here top products I have one of his bigger intercoolers on the Disco and it works!
Mike - thanks to anyone who gave this some thought! I was VERY intimidated by getting into the pump.