Chap on an Audi forum I use posted this.

The car started to judder around 2krpm and then loads of white smoke out the exhaust followed by more juddering and car struggling to move at all

I turned the engine off and had it towed to a garage...

RAC read the faults and it came up with piston failure on piston 4 (permanent).

Havn't lost any coolant or oil which gives me some hope!

I suggested the white smoke was consistent with water but he's adament it was fuel. He also said the oil has been contaminated with fuel which led me to suggest bore wash but he said the rings are fine, did try and say the fault isn't with the rings but the excessive quantity of fuel but not entirely sure he's all that sharp on the car.

Any suggestions?