Diesel Knock...

Afternoon all!
Got a lovely little old TD5 Defender here with a real strange problem..
Got lots of smoke & a hell of a knock on No4 cyl. Quick test, pulled the plug off the injector & the knock stopped. Bit of a clue methinks.. So! pop in a new injector, program it & hit the button. Starts as good as always with no smoke. Result! Knock is still there though. Am starting to get splinters under my finger nails now from head scratching... Checked the usual suspects, ecu plug etc.
Anyone got any ideas?..

Could be an actual mechanical fault, ie no injector connect=no load on the piston/bearings, reconnect and the load is there again?
Can you get one of the cheap boroscopes down the injector hole or maybe a dial gauge?