Thanks for quick reply. He did point out the offending relay, not sure I remember which one.
The back story is that I gave someone a jump start and the red lead crocodile connector made contact with a metal part close to the battery as I was manipulating the jaws onto the terminal. This of course created a short with a flash of sparks. When I tried to start the engine there was a buzzing sound coming from the relay box and no red light on the dash or glow plug light.

The buzzing , I was told was from the blown relay.

I'll have a look when it gets lighter, but I should say the the engine turns over, just won't fire up.

This buzzing noise seems to me to be the switch in the coil in the relay trying to make contact.

I took out and replaced each relay until the buzzing stopped. i think I've foundthe offending relay although have no multi meter. It is a C G Cartier PA66 Gf30 I can't see any amperage marked on it.

Does this sound like the problem?

I just done that with an identical one and it too started buzzing. So your right, sounds like a wire then.

On the side of the relay board there is a grey cover or something like that with two plastic press tabs one either end - presumably it pulls out. Is that another relay that is embedded into the guts of the board?

Yes that's the bracket, that holds the reset unit with the red rubber top. Blimey! you know these cars.