One of the storage heaters has had a broken knob for quite a while now. It's easy to work around as there's two on the heater, and I can swap one for the other when I need to adjust one of the settings that the broken knob is attached to.
Anyway, time to see if I can print a new pair of knobs as a fix.
The new knobs are in grey and have a similar but simpler design. I needed a pointer mark, so used a sharpie, but I could have put a recessed shape on one end of the cross piece, having thought about it.
This is them in operation, and they do fit and work pretty well: