I've got. TD5 90, and sometimes the engine check light and glow plug light won't light up on the dash when I put the key in which results in the car now starting.

It will only start when I take out the ECU relay and put it back in. Then the lights come on and the engine starts.

It's a new relay...any ideas?

Clean the contacts on the relay and give the socket a squirt of contact cleaner. If that doesn't work you need to start checking the wiring on the loom to make sure you have good contacts.