My wife owns a 2003 53 polo sdi which up untill recently starts fine. Now it takes upto 50 on and offs on the ignition for the coil light to come on so it will start. So far i have changed the coil plugs, coil plug relay, ecu relay, cleaned all the earths i can find. Everytime i play with it it gets better, than a few days back to square one. The only thing i have noticed is the diesel pump in front of the engine at the top primes when the coil light comes on, but not untill than.

Any ideas? as it's pissing me off and a bit dangerous in some situations