It wasnt actually the EGR valve, but on this one there is a vacuum actuated throttle flap next to the EGR valve. This had jammed shut. We still had a bit of fun trying to get it to start afterwards (Presumably because the cylinders were by now full of unburned fuel) but once it did it ran fine (Appart from initially smoking rather heavily)

Once it had warmed up, I was able to replace the other three glow plugs.

Job done! Thankyou