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Thanks for that rizla.
This problem has existed for several weeks, on and off. During that time we have not been able to identify any pattern or cause and effect, and believe me we have tried!
When the light has lit and the engine gone into limp mode, turning it off and restarting (sometimes immediately, sometimes after several minutes) has also sometimes ended the limp mode but not always and the light usually stays lit.
I take your point about the tank pump, it would be worth checking, although checking the filter on the engine pump revealed very little, not enough to suggest that was the obvious answer. I have had a tank pump fail on another 306 2.0 HDi I had, but that was a complete failure that stopped the engine dead.
One other idea might be a fault in the EMU, we have yet to have that checked.