Well, we have an answer, of sorts. The timing belt had jumped teeth! How i'm not sure - the car has only 74500 on the odometer although the belt is 6yrs old now, but the owner does like a 'spirited drive'.

Checking the timing this morning and cranking over by hand on the crank pulley i noticed that it didnt feel quite right. I suspect there may be valve to piston contact so may have to whip the head off to check later. This hole is getting deeper by the minute.....

Just hypothesising as to the cause, can these PD injectors fail and allow excess fuel into the cylinder possibly causing a hydraulic lock and causing the timing belt to jump or do they fail-safe i.e no injection at all? My concern is that 'just' refitting a new belt and retiming it wont be the end of the problem if an injector is leaking and it happens again?