Hi tom1979
A neighbour of mine had this very same issue on a TD5 he purchased revs not dropping when changing gear. This did not happen all the time mainly in the lower gears after using higher revs to create some inertia when climbing a hill say.
After a lot of searching himself and the use of a Nanocom he was unable to pinpoint the problem. So off to an expert who took it out for a test drive with a diagnostic tool connected was only away 5 min back at the garage he then adjusted the wastegate actuator to make it longer, another test drive again 5 min back at the garage said that should do the job and charged £30:00.
Sure enough the issue was sorted the previous owner had adjusted the actuator just a little too much.
The guy at the garage said he had come across the problem a number of times, also a remap can cause the same issues but not so easily sorted.
Good luck .