Belated reply sorry. I decided to call in at Powerline Shipley on 28 Oct and spoke to a guy there about the possible fault (engine management light on, EGR cleaned and blanked with plate with a hole so as to avoid ecu remap, map cleaned, but after £80.00 spent engine light still comes on). He was a mine of information, so I booked GT in for the following day, when he said he would do a diagnostic check while road testing.
As promised, he rang me a few hours later. Faults codes stored were: P0683 GLOW DURATION CONTROL, SIGNAL FAULTY, P1238 AIRMASS TOO HIGH, P0238 BOOST PRESSURE SENSOR FAULTY. He said he cleared the fault memory, road tested with RTS and checked live sensor data. Boost pressure and air mass readings were initially too high ( AIR MASS NOMINAL 1200mg, RECORDED 1691mg, BOOST PRESSURE NOMINAL 1.3 BAR, RECORDED 2.0 BAR). After a few applications of the throttle under load, the readings produced were correct. This indicates a fault within the turbo vnt mechanism. All other sensors including egr valve and turbo wastegate actuator control solenoid were found to operate correctly.
In short, he thought it was probably sticking turbo vanes due to regular short journeys. I've done a trip to Cheshire since as well as regular weekend runs out, and the engine light has not come back on - yippee!!!
When I called to collect the car, he was very taken with the GT - lovely shape, lovely colour, plenty of grunt (but they can't decide where to put a number plate!!).
I bought some Stanadyne Deisel Fuel Treatment while there and have added some to the tank. In all it was £80 plus VAT for the diagnostic and £15.50 for the Stanadyne, which when I add in about 45 minutes of his time chatting/advice and no recurrance of the engine light issue to date is, I feel, very reasonable. Can't recommend highly enough.