Exhaust Warning Light / Loss of Power

I have a 2006 1.9tdi Passat. It has run without a hitch for the two years I have owned it. I work seasonally abroad and the car was parked up over the winter. I am now in Austria for the summer and before taking the Passat over the channel I had the foresight to have it checked over by my local VW dealer. All was fine.

The other day I noticed a severe loss in power but put this down to the steepness of some of the roads I was driving on over the Alps.

Today, however, the Exhaust Warning Light has come on.

I have booked it in at the local VW dealer who will look at it on Monday as they close over the weekend.

Does anybody have any ideas as to what may have happened?