Best way to insulate a draughty Victorian sash window without replacing it?

We have a couple of original sash windows in our 1890s terrace and while they look great, the draughts coming through them in winter are pretty brutal. Heating bills have been creeping up and I want to tackle it before next winter.

The obvious answer is to replace them with double glazed units, but I'd really rather keep the original timber frames if possible. The character of the house matters to me, and the council has made noises about the conservation area before, so I want to tread carefully.

I've been looking at a few options: brush pile draught strips fitted into the runners, secondary glazing panels, and that rope caulk stuff you can remove in summer. Has anyone here actually tried any of these on period sash windows and had decent results?

Mainly wondering whether secondary glazing is genuinely worth the cost and effort, or whether it ends up being more hassle than it's worth. Also curious about pitfalls when fitting draught strips, like damaging the original timber or making the sashes stiff to operate.

Any experience or advice from people who have been through this would be really appreciated. Happy to report back once I get going on it.