Many times one needs a much longer shaft on a pot or tuning capacitor.
I have come up with several ways to do this without an insulated or special coupler.
1. If the shaft is long enough, cut threads on it with a die and add an "all thread" type coupler. Use a threaded shaft to screw into it, and Threadlock all the threads.
2. Take a small sleeve and drill 2 holes through it. Then drill the shaft and the extension and put a pin through them, or thread the holes and add set screws (Or for you in the UK, Grub screws). Sleeve can be metal, Delrin, or even metal, Plexiglass or plastic rod drilled out & threaded for set screws.
3. Solder the 2 shafts together with a piece of brass tubing as a sleeve. Use lots of flux and a huge iron as not to hurt the control or capacitor with overheating with too small an iron.
A small piece of automotive fuel line- rubber- can be used provided it has a really tight fit; to couple the shafts together.
It likely won't stand a lot of torque, and likely wont last too long. However, for a control that's seldom adjusted, it might work just fine. Sometimes heating the rubber, and freezing the shaft can let you use an undersized piece and at room temp. will fit really tight.
Feel free to add to this list. These are just a few I thought of.