The Heatmiser boiler control is the zero volt contacts labelled 'C" and 'NO' on the 'Boiler' terminal block.
You would need a connection from the incoming Live of the boiler to 'C' and then a separate connection from 'NO' back to the boiler switched live Lr.
If a flex is used, both conductors can be live, so you would sleeve a blue wire with brown!
Greenstar boilers (if that's what you have) seem to have have different mains terminal arrangements depending on the model.
If you're not clear this might help -
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