I had the same in Pug 406 HDi and it was as if i had run out of fuel. The engine seemed to have no power and my speed was slowly decreasing. I was doing about 90mph at the time. Turned out the fuel pump (driven by the cam belt) had seized which ripped the belt off.
The valves were punched back up into the head, which broke all the followers. This was considered lucky as this released any pressure on the valves to push back downwards, so the valve springs kept them "safely" up in the head away from the pistons.
Take off the £600 for the fuel pump (yes... that was £600) and the rest of it cost £700 parts labour for valves, followers, gasket set, head bolts, belt, tensioner, waterpump, bottom pulley.
AA didn't try and start mine as I had already spotted the belt hanging down as most of the belt cover had been smashed off in the forces occuring underneath, although I did not hear a thing when it happened.