Sime 100E boiler:How to replace the knob /spindle on the potentiometer?

Hi

I have pulled the knob off the central heating potentiometer on the boiler and instead of just the knob coming away it has also removed the spindle as well. I can't get the spindle back in to the correct position with the result that the central heating is only coming on for about 10 mins -goes off then comes back on again 5 mins later - We are all freezing.

I should add that the spindle has a little groove cut in it at the end which goes into the potentiometer

Any advice on how to locate the spindle correctly gratefully received