Hi everyone, progress has been made and the problem is solved. I topped up the gear box with 500 ml of gear box oil while engine was cold and moved the gear stick from park to 1st and then up through the gears before and after topping up. Took the car for a run down the dual carriageway, but the problem persisted, with the speed topping out at 72 MPH. Not really fancying checking the fuel pump as a next step,I pulled over and had a look at the tuning box. There isn't a name on it, so not sure of the manufacturer. As previously mentioned the box used to flash when the engine was running, but had stopped flashing recently. While I was checking for a switch on the box , incase it had been turned off, the wires must have moved and the light on the box started flashing briefly. Further wiggling, application of insulating tape to hold the wire in place and bingo the box looked like it was working. A test drive then confirmed that all is now well. In combination with new hoses, sensors, filters etc, the landspeed record for a P38 Diesel could be in reach. The improvement is so pronounced, that I will have to tread lightly on the throttle, as only half the peddle pressure is needed for steady acceleration.