After doing lots of reading up on the 1.6Dci engine I think that it could be DPF Regeneration, apparently the DPF can be cleaned a lower speed on these engine's including normal round town trips as effectively as motorway journeys, although it would be nice to know that it was trying to do a regeneration, as far as I have found out the DPF light comes on when the filter is partially blocked and by that point it's probably too late. Would be nice to know what the actual requirements are for a DPF cycle to start and if there is any tell take signs that one is in progress before switching off the engine. Going to take mine out for a run this morning at 50mph and try to keep the revs at 2000rpm, I think it needs 10-15 minutes for a regen so I just hope it does one haha!