Our 1.9tdi (130) has the check engine light on but not consistently. For example, it went off on a 5 mile journey this morning after a few yards but was back on for the whole of the return trip. Engine seems to go a well as ever. Should we be concerned? What is the cause likely to be?

Needs to be scanned for codes really before we can say as that could be anything

As above, needs to be scanned.