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2003 td5 110 csw occasionally decided to cut out whilst driving or refuses to fire from cold.

Symptoms include no fuel pump priming, no coil light and temp gauge reads hot even when cold. A mixture of shunting the front seat or cleaning relays resolves it.

I've narrowed it down to the main relay (third in from left under driver's seat). If I move it or apply pressure it fires into life.

Tested relay which is fine, threw a new Tyco in anyway and it's definitely not the relay.

Will it be w bad relay plug/loose connection or is this likely to be an earth problem?

Well, if you can solve the problem by wobbling the relay or rattling the seat about it suggests that it's the connection between the relay and the wires in the relay holder. So I'd start by putting electrical cleaner on everything and pushing the relay in and pulling it out again a few times in the hope of cleaning it up. Give the wires a tug where they go into the back of the relay holder - I've had problems where wires fatigue through where they go into the crimped-on terminals. Inside the relay holder there are usually terminals that look like female spade connectors that have a little tab on them that keeps them in the connector housing. Sometimes it's possible to slide a very slim bit of metal down the side of them and depress the little tab and get them out to clean them properly, check the wire is securely crimped in with good continuity and maybe give them a squeeze with a pair of piers to ensure that they're gripping the male tabs on the relay tightly.

You're lucky it just seems to be the relay holder. When my TD5 did this it was the ECU itself which was at fault.

My dads TD5 had the exact same issues. It hasn't done it for a few years now, but the long cranking time seemed to be caused by low fuel in the tank, iirc it only did it when the fuel level was below a quarter on the gauge, but we never "proved" it, was just an observation. Our mechanic changed the injector seals as well. Might be worth checking these if the problem continues.