Sticky passenger door mechanism

The passenger side door mechanism on my SWB 460 has become very stiff/sticky. The central locking/opening is doing its job, but I reckon the mechanism in the door is just too tight to be moved. Right now I open and close the pass side with the key. It works fine - though it is stiff. In normal use, the c/l will lock all doors (pass side too), but will not open that pass door.

I'm good to go to take it apart and give everything a good clean. Bit I'm a bit nervy about what bits I would do well to leave alone...

So. Anyone who's done the job got any words of warning, advice, best practices - I'd be quite grateful to hear them. I'm a bit ham fisted, so anything of a delicate or (mind you don't lose this spring or that clip) would be well received.