Got the chain off - and very very stiff it is. The master link was really really tight , not due to rust but due to a bit of metal deformation on the pins. No idea how they managed that, or if it was just a really old chain.
Decided not to clean the chain, but get a new one.
So new RK 520 chain on the way.
Cleaned the back sprocket and started on the front sprocket area.
VERY gunged with old crud and oil and lube.