Just to update anyone that's interested.

Took it round to mark_24v_wiring this morning who plugged it in (EML light come on during the journey over!) He looked at the fault code, then did a live read.

Turns out the upper and lower o2 sensors are all over the place and by the sounds of the exhaust etc and the readings, it looks like the cat has gone. Bugger!!!!

Not a cheap fix, but apparently it will sort out the bad running, shit tickover and when done, it should get back to normal power!

The fuel return is all good too. So thanks for the replies and help everyone.