03 tdci doesnt start on key

[aj1soad]

just had the engine on my 1.8 tdci connect taken apart to replace a piston and now that its been put back together its got a problem. it wont start with a key but it will if you jump start it and it will tick over all day no problem but once you try and drive it a few metres it will cut out and the heater plug light comes on. ive had it connected up to a computer and it came up with a fault on the fuel pressure regulator. i bought a brand new one of these and put it on and still the same. so next i tried another fuel pump and its still the same. any ideas whats wrong as im fed up of buying parts for it now. thanx