Well.. thought ild post an update since i found the source of my troubles...
I needed to get the codes read, and Amber Garage came to the rescue, although not that local to me, i limped the car there in torrential rain (hoping it didnt conk out on route..).. They found several fault codes, listed below:
DPF temp sensor error and O2 sensor (P2A00) range error,
some older codes (i think) relating to glow plugs and a familiar brake pressure sensor one (that seems to come and go)..
So next day i got under the car to locate said temp sensor, its upstream of the dpf and a 2 wire (must be resistive) sensor, theres a 2 pin plug near the sensor, so removed this and checked the resistance, (expecting open circuit, but read 206ohms..also checked the resistance of the other temp sensor just after the turbo and this measured a similar resistance, so confirmed it looked ok), so i then traced the fairly long cable running up behind the engine, and it seemed to take a bit of a non direct path, so im looking with my torch and i see a frayed wire and some burned cable wrap, (cant believe it.. looks like this is a simple broken/damaged wire,..) so repaired and slightly shortened the cable with several heatshrink layers on top, then checked the connection back to the ECU on the airbox.. once reconnected and got the expected 206ohms so all looking good..) put everything back and took it for a run, cautiously.. seemed to be running better, and more like its normal performance,, about 5 miles down the road the 'soot filter full' message came up (expected as it had done probably 100miles in limp mode prior).. stopped in a layby and as i pulled away an enormous sooty cloud exits !.. that must def be the DPF regen... ) took it for another 30-40 mile run, and added some liquid moly dpf cleaner, so to assist with any dpf issues. Next day i had to do a 500mile round journey but getting it fixed just in time i was chuffed !.. car ran superbly def had its mojo back.. added some fuel system cleaner also.
So the moral to the story is, check those sensors first if you dont have any way to see the actual values being sent..
So in the end, it wasnt any of the forum favourites, and turned out to be a nice easy one to fix.. ive since been monitoring some other parameters using the torque pro app, and the pro version seems to open up quite a few new parameters that the free version didnt, such as fuel rail pressure, fuel flow rate, egr %age etc, fuel rail pressure (5000 psi@tickover) and fuel flow switches to 0 when you're coasting downhill etc, if your having limp mode problems is def worth measuring the 2 temp sensors on the exhaust, the code relating to the O2 sensor seems to persist, so i replaced it with a bosch one who appear to be the oe supplier, since then you can see the voltage flipping between 0.5 to 4v range, where the old one seemed stuck at around 0.3v, so it think its working now, but the O2 sensor code seems to be there despite a fault code clear with torque.. The EGR seems to change between about 40 and 100% and then often at 95.9%, so not sure if this means its working as expected..egr error seems to show some random figures between + and - 100% so im not sure thats correct or a real value?