Thanks guys - much appreciated. I've looked at a host of videos now, all very interesting (well, most).
I was wondering about the different ways of putting something poking out of each end of the hand rail which then enters into the post (from dowels to tennons to expensive swively bits of metal). However, with the posts looking to be well and truly fitted, and I'm rather reluctant to try to unfit them, the basic problem of how you put in a handrail between two fixed posts is that if both ends have a pokey-outy-bit, of any form, I won't be able to position the handrail between the posts.
I think I will use a "screw it in form the far side of the post" - strategy, but put a cross dowel into the hand rail to give the screw something other than end-grain to grip.
Insane?