So you are getting this noise, with no load on car (car in neutral)? You did rule-out the accessory belt and any pulley or bearing associated with that belt?

 

This still sounds likes rod knock or piston slap to me. But playing this guessing game over the net at this stage isn't going to solve the issue that needs to be 100% identified and dealt with. Piston slap isn't anything you can do something about without rebuilding the bottom end, but a rod bearing could be if its caught early enough. But continuing to drive and experiment with recreating the sound without a resolve is going to continually diminish your chance at a relatively painless fix if you do have a rod bearing beginning to go out. If I were you, my next plan of attack if you haven't done so already, is to get in touch with a trusted mechanic and have him listen to the sound and agree to remove the pan and inspect your rod bearings if he agrees there is a strong suspicion of damage there, with the intent to assess the status and condition of wear for bearing replacement WITHOUT a total rebuild. If the bearing is worn but not spun, this could be fixed for an indefinite amount of time without removing the crank and pistons, doing a full bottom end overhaul (engine and head removal etc etc) Consult with your local sub forum if you need some recommendations on local mechanics if you currently dont know any.

 

I have arranged for it to be done at a well respected garage. Only problem I have is I live far away from there and will not have the car back for 2 or 3 days whilst they order in the correct sized bearings based on what the crank codes are.

Is it best to go with the OEM bearings or can the one size fits all ACL or Kings bearings be used instead?

 

Oil pan pulled off, bearings have no wear at all on them. Though they aren't the stock bearings, I believe they are the Kings standard sized bearings. The mechanic said the oil pan looked extremely clean, as if you had opened up a brand new engine. This is probably explained by the new oil pump the previous owner had put in 6k miles ago. I don't think the non OEM bearings were put in then though.

Nasty rattle still present when engine is above 2200 rpm, worse when getting on the move and can just about be heard when doing 70mph in 6th. I wonder if it has something to do with the clutch or flywheel.

The noise it makes when cold that sounds like knocking I have put down to piston slap.

 

Drive it hard until it fails or can be diagnosed properly. At least you will know where you stand. Engines make noises, the bad noises evolve rapidly.

Lean on the engine, and the issue will become apparent.

 

I will carry on driving it until something blows up.

See this video for the best explanation I can give. Same noise with the clutch in or out. Releasing the clutch after you change gear changes the tempo.

https://youtu.be/D4tKyMcupcc