Actually Nigel I was in bed last night thinking about what it could be and I am CONVINCED that it's a boost leak..........

Why? because I had exactly the same symptoms when my turbo to intercooler pipe had split (under the pipe where you could not see it)....

Baffled me for ages..........as it would (eventually) still make full boost but was very sluggish to build up boost.

I now have an alloy "plug" with an airline fitting attached, borrow it if you want, or come round to my place and we can test it . You disconnect the air filter (90° hose) and fit the alloy plug to it, then do up the jubalee clip, next connect up compressed air line at the SAME PSI as your maxium boost (eg 28 PSI).......If there is an air leak - it will be very obvious AND easy to find