Disassemble spring-assisted sash window

The main source of heat loss is not through the glass but through the gaps in the window frame and where the sashes rattle.

There are various ways you can reduce the air flow though the window.

The cheapest would be to install some cling film secondary glazing. This one looks like it just tapes up over the windows to keep the draughts out. Other systems use heat to shrink the film to size.

You could hang some heavy curtains. Make sure that they reach all the way to the floor and are plenty wide enough so you can have maximum gathering. If you line them as well there'll be two layers of fabric between the window and the room.

Next up is something like this from 365 Plastics. It's sheets of perspex cut to size and fastened to the windows with magnetic strips. We have this and it works quite well. The main drawback is storing the sheets in the summer.

A less visible solution, which you might want to use in association with one of the above is to renovate the windows with something like one of these draught proofing kits (others are available!). With these you replace the staff and parting beads with the supplied parts that include brush pile that forms the actual draught-proofing.

Is it DIYable? Yes. No. Maybe?

You may find that you start the job, everything works out just right, and you complete the work in record time and under budget. More likely, you'll run into a bunch of unforeseen problems, take 5 times as long as you'd expect, and end up way over budget.

The best way to tackle DIY work that you are not sure about, is to have the resources available to quickly fix mistakes. In your situation, it might be a good idea to purchase a replacement window before you begin. If you damage the old window beyond repair, you'll simply have to install the new one. On the other hand, if the repair works out, you can return the new window you purchased.

It also never hurts to have somebody with knowledge in the type of work you're doing, so if you run into trouble they can offer guidance.

If you come across something you are really not comfortable with, there are always professionals available to do the work for you. You might also be surprised that they charge less than you might think (not always the case, but does happen).