£1.39 groovy knob purchased:

It has a round spline with a grub screw to attach it to whatever it is going to control. However, the shaft of the tap is square and actually lowers with the rest of the mechanism leaving the grub screw clutching air. I needed a round extension to the shaft with a square aperture. Step up the rawlplug! i cut a couple of the plastic pieces shown so that they fitted in to the knob.

I then used araldite to fix this to the knob and tightened the grub screw in for good measure resulting in:


which I think looks pretty cool. I just should have checked the height first - it touches the top of the kitchen lid when closed - a couple of mm shorter would have been better. I may cut a bit off the bottom of the rawl plug as this is not glued to the tap - just fits snugly.
I also replaced the tired microswitch with one from maplin and to give the tap actuator more chance of pushing the arm of the switch I bent the lever of the switch back on itself rather than cutting it off. worked quite well.
I also chucked all the relevant pieces into a descaler for a while to make them nice and shiny. Just need to hook it up to electric and water now!