Craig a common problem with VW 1.9 TDi older engines was air mass meters which packed up after 40-50k miles, people used to come to me regularly saying they wanted a box or remap for them because the car was painfully slow, this was because the duff air meter wasn't letting any fuel in just air so no power but mega MPG. Tuning these cars made no difference only fixing the problem solved the power issue, in fact after replacing the air meter the customers felt like the cars were already chipped, the difference was that noticable, then add a box or remap and boom mega power!!
I think it sounds as if your cars problem is the opposite of the VW problem, it sounds like yours is over fuelling in a big way which is or would be causing plooms of black smoke but because you have DPF it's catching all this smoke and blocking up very quickly and can't regenerate quickly or often enough so the light comes on because its blocked. This is not a DPF fault, it's doing its job but is being overwhelmed by the amount of soot its having to deal with.
The EGR valve is used to re-circulate exhaust gases around the engine to re-burn them but if it sticks can cause a massive carbon back up and smoke galore. I know Landrover had alot of issues with these on the Range Rover Sports originally and it would cause shuddering and rough idling etc.
If this is the problem then by remapping to increase fuelling you'll be adding to the problem not helping it. I would ask the dealer to remove the EGR and clean it or check if its functioning or if possible renew it. I can't think of anything else that would cause this problem but not throw up an engine fault light.
I can't say for 100% certainty that this is the problem but its the first place I'd be looking with the problems you're describing shuddering/high fuel consumption/DPF blocking.
Hope this helps you resolve it. The reason the garage probably can't diagnose it is that often EGR valve problems won't bring up fault codes or engine management lights so the dealer assumes there is nothing actually wrong. It can be a tricky thing to diagnose. Incidently the VW air meter problem also didn't bring on engine light or fault codes.
To be honest the whole DPF thing has been a nightmare and a real headache for dealers as anyone not driving regularly on the motorway or at least doing longer journeys will have re-generation problems, this is especially bad for taxi drivers and older folk just doing short trips. I know for a fact that most of the dealers I supply to have cars in every day to be forced re-generated.
Cheers
