In reply to Tom Ripley Mountain Guide:
buy a diagnostic code reader, read codes, reset codes, eat, sleep, repeat .
as already suggested, my suspicion would be the pressure valve of the turbo is probably partially gummed up with carbon. an inefficient turbo won't immediately be obvious; might need a proper diagnostic test - with rolling road, not just reading error codes.
but, your van could drive for years with a slightly crappy turbo, maybe just a bit less fuel efficient. if the pressure valve is seized it'll still work and the ECU should cut it if it over-boosts (at high revs/load): then just switch engine off and back on again
a 2nd hand reconditioned turbo might cost you £4-500 or so, and fitting on top of that.