So spent about an hour under the old girl, in the cold, in the nearest national park some didn't p off the neighbours with her running and the occasional rev.
Fluid is fine - no evidence of burning or coolant contamination.
So I had my fav long screw driver stuck to my ear most of the time listening time internals and the noise is 100% from the bell end... It's right at the mating surface - and the noise has gone from a chatter to a matalic rattle as if you had a piece of stainless on top of an anvil and went over a fair whack of ripples...still predominately when in gear.
No slipping as such but power loss for sure.