Espace turbo fault

Hi i have a strange fault with an espace 2003 2.2 dci, it will only boost if i tickle the accelerator and its rolling, looking at live data map pressure is normal atmospheric pressure at tickover when i rev it it goes up to around 1200 to 1300 but drops off, codes are 1815, turbo looks new no play, i have replaced boost valve, no change, no smoke, no clues anyone have a suggestion other than matches, regards Rob