Been after a pair off heater knobs for a while now so when a pair came up on eBay that I could actually read the writing on them I decided to bid to buy them.
To remove them you need to push a small screw driver down a small hole in the side of the knob to push and release a retaining pin pin in the shaft.
The new knob just pushes onto the shaft until it clicks into the spring loaded pin. Be careful not to lose the pin whilst the shaft is exposed.
MGB tips mods and maintenance 