L series random knock knock tap tap

Eh up,
My hippo has been making some strange noises over the last couple of weeks, so I have done a bit of invest but could do with some advice.

Randomly there is diesel knock or a loud TAP TAP TAP etc before returning itself to normal.

Its got fresh oil in, ive checked #1 injector plug and wiring, timing belts all correctly tensioned and timed, all lecky plugs clean, millers in the fuel.

Its like the timing keeps moving around so im thinkng of cleaning the Crank Sensor or maybe even changing the #1 injector (i've got a spare set).
I dont want it to be the pump.......
Sometimes it kicks out some smoke with it and a bit sluggish so thats why I think it timing related.

What do you guys (or gals) reckon? Anyone had this before?

I have no idea,but if you think its timing related why not change the timing belt and tensioner ?

All timing belts and tensioners are less than 6 months old, I have rechecked them, there associated pulleys and they are spot on.
It starts on the button and sometimes can go 20 mins before the the knocking starts, or it can do it immediatly after startup. Then it disappears as randomly as it appeared.
Sometimes it will knock for 10 mins sometimes for 20 secs.
When it has an episode its especially noisy on the overrun.

Bizarre, and its doing my swede in!

Changed the #1 injector no change, think will try a replacement crank sensor. Cleaned all earths, connectors again no change. Started first time easily with no plugs, so can discount the static timing.

If all that fails, clutching at straws, its going to be a hydraulic tappet flush and oil change.
Even though the oil is only 1 month old....

Try the cps ,you might get it out by just removing the heat shield.I went by the haynes after attempting to remove it with the exhaust manifold still in place.Easy peasy once the manifold was out the way.
Try to pick one up from a breaker and take note of exactly how its bolted to the engine.

Thanks for the info so far woolfy, I got a cps on order (tenner off ebay!)

Right chaps, the saga continues:
After pratting about all day I have found some more info for starters there was 2 different noises.
One has been solved by discovering that the 3 camshaft damper plate securing bolts were loose, one was about to fall out into the cambelt!!!
Obviously tightened up and it immediatley stopped one knock, also engine ran noticeably smoother, suppose the L series super sensitive timing musnt like a damper knocking it all the time!
I then changed the remaining injectors, its now much smoother and almost smokeless!
I checked the injector part numbers online, my old set were from a rover 820 and about 1/2" longer. My new smoother, less smokey set are for freelander.

However my new found engine smoothness highlights the very loud tappety tap, in particular from cold and overrun of what I now believe to be a hydraulic lifter messing about. Seems to be doing it more from cold/after letting it settle for a couple of hours.
I gave it a good italian tuneup and the engine is almost silent. Will have to see what the morning brings.

Now does anyone reccoemend a flush or method to clean the lifters in situ? I have read into replacing them but the lifters are matched to the head and have been told I cant replace them.

Thanks in advance