Td5 cuts out - only restarts when glow plug light comes on!?

My TD5 starts first time when cold, but cuts out during the journey. Then it won't restart until the glow plug light comes on. I have taken it to our local LR specialist - couldn't find a specific fault but changed the wiring loom to see if it helped. Worked for 50 miles then started cutting out again. Still only restarts when the glow plug light comes on - I don't need to wait for the light to go out. Any help would be much appreciated as it has been off the road for weeks now! Thanks.