Despite the interestingly shaped intercooler, I couldn't really see any major leaks which would lose that much boost. So I rang my local landy mechanic to see if he had an intercooler and explained my issue in detail. He said - "fuel pump at a guess". He told me to take the filter off and check for a good jet of diesel etc...
So I thought, let's try the simplest test. Ignition on.... silence....
Bollocks... why didn't I think of that in the first place!!!