Whilst driving the other night, there was a noise of something clunking and rattling its way loose from the engine bay and the tell-tale sound of something being deposited on the road. Accompanying this was the loss of power steering, the battery light coming on and a message on the computer saying 'Electrical System Service Required'.
Without even looking it was obvious that a idler or tensioner had given up and disintegrated and left the vehicle, leaving me without alternator, A/C or power steering drive.
VERY luckily, it had not taken the timing belt out, which I have heard it can do on this car by getting snarled up. I managed to drive home without stopping. Steering was very heavy and I turned the lights out to minimise battery drain. Luckily, being a diesel, there's not a huge drain on the battery whilst driving.
When I stopped, I had a look and the belt was just sitting in its normal position except it was just standing off all the pulleys, as it had of course lost its tension. There was nothing except the central hub of the bearing and bolt left where the idler was.
Fitted new belt, tensioner and idler the following day. See the little guide here.