Are you sure it is a top end problem if so i would still say its a cam or follower problem have you visually checked these if not take each tappet out and check the bottom of the tappet for wear or if you find any of them are a little stiff to pull out check in more detail as this normally means there is sighs of wear, if not try and press the (little button) on top of the tappet down these should be quite stiff. but when you say

"Doesnt really do it on idle (but you can tell the sound is there), more when you accelerate and back off.
The car runs fine other than this, so I cant figure it??!!

I would say it is big end bearings the only way of checking this is to take the sump off and remove the bearing caps and have a look, remove each one and make sure they go back exactly how the come out it will be easy to identify. If you live by worcestershire bring it down to my unit as i have a ramp and you can take it off there 'no charge at all'. but dont drive it until you find the problem my bigend bearings totally give up in 40 mile of knoking and sent a piston through my block