So, I have now got an update regarding the mapping of my Golf.
I had the ECU re-mapped and the car performance increased quite noticeably. The running cost (fuel consumption) doesn't appear to have changed much, a touch slightly better but nothing significant.
But after a short period I started to have problems with the DPF (diesel particle filter) blocking quite quickly. This is a filter that is part of the exhaust system which basically cleans out the soot from the exhaust gas. The car is programmed to regenerate the filter, basically when the car gets to temperature the car's management system alters the fuel mixture and injects fuel in the exhaust so that the soot can be burned away at high temperature.
It turns out that my car had a faulty EGR valve (exhaust gas recirculating valve) which is also part of the exhaust system and is all linked in with the DPF and controlling emissions.
The car began to run terrible and I found I had to take it on a long run to clean out the DPF on a regular basis (at least once a week) to sort the problem. So after living with this for a month I decided enough was enough and got a friend of mine to see what he could do.
My options were:-
A. Replace both the EGR valve and the DPF (at a very high cost, in the thousands).
B. Have the car re-mapped again to omit the EGR valve & the DPF and modify the exhaust.
So a piece of advise, have the vehicle checked out first before you commit to performance modifications. Modern diesels, due to emission laws are a pain in the butt!