Ruston there's no easy way to get at the back of them either so all the soldering has to happen in the visible areas.
My solutions, which isn't sophisticated but does seem to work OK, is to tack solder the knob in place with ugly blobs of solder applied with the electric iron. Then blob lots of flux over the resulting mess. Finally give this a shot with the small blowtorch.
The torch heats up the knob and surrounding metal faster than the rest of the bodywork can absorb the heat away - something the electric iron can't quite manage - and so the solder metals and is carried into all the places you want it by capillary action.
Result: Handrail and fixing all held in place with no cleaning up required.