Don't buff the surface. Just wipe it on and leave it. If you aren't over-applying it then there is no need to buff.  

I'd been using 2 microfibres cloths; 1 to apply and 1 to buff. What would you recommend using instead?

There's a few things that can do it though. Oil on your hands, too much Easylease, and only using one application cloth for all the coats. You should always wear gloves, only use a small amount of release agent (if it doesn't evaporate within a couple of seconds of wiping it on, then it's too heavy), and always use a new cloth between coats. 

Photos of the sheet are above. The cloudiness is reasonably clear in them.

They are slightly cloudy marks that do look as if it's where I have moved a cloth over the plastic sheet (used as a flat surface) when applying release agent. Would that be indicative of over application of easylease?

I don't think it will have anything to do with the Easylease (except for potentially over-application). Easy-lease won't go cloudy over time, it may just lose its release properties. 

I think I'll try some newer easylease and see what happens.

I have been away with work a lot so not done much since then but made another flat panel last week. This time with a soric core. The surface finish on this is much better but it is possible to see some cloudy streaks in the finish. How is the best way to minimise these in the future?