[Scott.T]

I would wait until Vince has had a good look before you jump to too many rash desicions.

It may be tappets or, as mentioned out in germany, it may be the cam belt tension has gone soft.
this makes a nasty knock as the plunger bounces of it's resting point due to the self priming plunger oil seal failing.

Mine done this a while back and would give a nasty knock noise on start up as it bounced.
Vince swapped it for a new one that I provided when I had the belt done.

I have another new one on the shelf ready to fit when the belt next gets changed as they can often fail after a cam belt change. The plubger has to be compressed and a pin insterted to old it, then when re-fitted the pin is removed to allow the plunger to reposition itself. if not done slowley and vertically the pil seal can be damaged and then after a while give the above symptons.

it could be as cheap as £160 for a new tensioner plus labour to fit new belt and/or recheck belt has not jumped a tooth.
Vince has also found in the past broken cambelt pullies or guides that also sound like big end knock.