Problem 1.8 TDi engine randomly cutting out but starts fine

Hi all.. We have a 2000MY Focus Mk1 1.8 TDi estate that has a habit of running fine for journeys of as much as 12 miles but towards the end of the journey or the next day will run for say 3 miles then engine will just die while travelling at a constant modest speed, anywhere from 20 to 60mph usually in 4th or 5th so revs about 1000 > 2000rpm. The immediate solution is to engage clutch, coast, switch off engine, switch back on immediately and drop the clutch. This succeeds in restarting the engine again. It will then behave for miles or cut-out again within 1/2 mile or so. We have done 500miles since the private purchase of the vehicle. The previous owner had had similar issue and had had the immobiliser Engine Management Unit reconditioned. The previous owner suggests changing fuel filter as he had done so but with a pattern replacement. Our mechanic suggests it is the fuel pump solenoid. It has never cut out at tickover. It has never failed to start first time. It has done it once with the engine cold after travelling 100 yards. Last month it did 4 days of short run local commutes without incident. During the last journey where it had cutout around 30 times in the 60 mile round trip it did seem to be exhibiting a few moments of momentary power loss for a single firing cycle but without actually cutting out. When it does cut-out the preheat lamp flashes until the ignition key is cycled. Any ideas?