I have some railings kits, picked up cheaply in a DIY shed sale: horizontal
softwood rails pre-drilled for tubular metal spindles. They go together
very nicely when fitted on the level, but I'd like to use them also on some
steps, which will mean re-drilling the 17mm diameter spindle-holes at an
angle.

I think I'm going to have to rig up some sort of jig to ensure that the new
holes are at a consistent angle, but I suspect that asking a bit to bite
accurately not only at an angle but also into an existing hole is likely to
prove tricky. Any tips on doing this? Many thanks.