Replacement Balusters

And how are these made, please?

I had several broken or missing when I bought this Victorian house. Got a local woodworking shop to turn up some matching ones. Wasn't that expensive. 40 years ago. But have you checked local timber merchants? Several in London do supply this sort of spindle now. But may not be identical. Or a re-claim yard.

Personally I would attempt to make them by hand, I reckon a decent enough replica could be made, I dont have a lathe. but its not the first time I have used my bench grinder to get rough shapes then sand by hand.

Not seen any like that before... however, some options:

Make the missing ones - could be done on a bandsaw.

Take a mould of an existing one in latex compound, then cast some replacements in resin.

Draw one up in a modelling program and use a CNC router to produce them from timber.

Buy a bulk pack of new spindles and replace the lot. (probably not that expensive if you are happy to fit them).

Buy anything close ish and fix the missing ones - chances are any prospective purchaser will not notice, and even if they do, its unlikely to be a deal breaker.

I was thinking more of a router jig to rough it out, using a removed spindle as a template, with a bit of hand finishing on the smaller features.

Yup, you could certainly get the broad shape with a template and a guide bush. The smaller features would need touching up by hand or with a set of small cutters in the router working across the width rather than along the length.