I have a works trip next week.. (Right now??? Zzzz! ) so I didn't want to dismantle too much and leave it all in bits.. but I removed the cam-belt cover and sifted through the wreckage.
The culprit is definitely the water pump - without question beyond all shadow of doubt.
The tensioner is still tensioning and spins freely, the belt pulley is immaculate and spins freely.. actually both these still have that "new" bit of stiction in their bearings.. they don't "whizz" round like old bearings...
but the water pump pulley can be rocked backwards and forwards almost 1cm out of it's axis.
What looked like it happened is that the pump bearings (?) collapsed allowing the pulley to rock in and out... This eased the belt off the pulley slightly so that it was only prevented from coming off when it touched the cam-belt cover.
Eventually the belt wore through the cover and this allowed the belt to poke out and then jam.
The belt itself, despite being shredded to half its usual width, was still driving all the pulleys and still hanging on it there till the end. Can't fault the belt or the tensioner or pulley.
Water pump is a Bugatti job, bought from Shop4parts in May 2011... and managed 57,000 miles (36k of its first cam-belt plus 21k of its second). I didn't remove it yet to look at the internals (it uses 6mm hex bolts and that key has gone AWOL from my tool kit.. ) but it still spins freely and makes no noises.. just the pulley rocks about on the shaft. No other sign of any issues (leaks or shards of metal).
I'll only be able to take the head off in 3 weeks time (cheers boss!) so crossing my fingers for just mild carnage until then..