Right!, i've had a good look at the crank pully. the section where the aux belt sits can be moved backwards and forwards (to and from the engine) by hand approx 2-5mm. is this normal play?
I very much doubt it!
The pulley is in several parts, bolted together.
There's a central pulley bolt - this MUST be TIGHT! It isn't as big a bolt as many engines use. Make sure yours is TIGHT! RAVE says it should be 455 Nm / 336 ft-lbs torque, which is colossal.
That needs a six foot scaffolding pole.
I somehow find it hard to think this bolt has come loose!
Rave ....
6. Fit new crankshaft pulley bolt and tighten to
455 Nm (336 lbf.ft), using a torque multiplier.
There are three smaller bolts around that which hold on the TV Damper disc. I think TV means torsional vibration damper. These bolts too must be pretty tight so this damper doesn't flap around and VIBRATE .... . They should be 80 Nm / 60 ft-lbs torque, still plenty tight.
Pray that these are loose! If so ....
also i've noticed a diesel leak around the fuel filter which appears to be comming down the pipes rather than the filter itself leaking. Would air in the fuel lines cause the engine to run rough/tappy?