Freelander 1 Engine light on and rough idling?

The engine light on the wife's Freelander periodically comes on. I have also noticed that when sitting idling for a while the revs seem to change from 500 - 750rpm, eg it does not idle constantly and it seems to rev on its own.

Anyone have any idea what this might be?

I was thinking about purchasing an engine diagnostic machine in an attempt to get the fault code?

Thanks

And the engine is?

Apologies. It's the BMW TD4 engine.