For those that have the same setup as me, in the end I replaced my sensor with a hw thermostat as I was receiving an Ad message, when the boiler could not see the sensor.
I used the following diagram and after a factory reset all is good.
I just installed a Nest thermostat E to a Worcester Bosch Greenstar 30cdi.
Pdf attached to this post, apologies for scruffy writing.
Hope it helps someone.
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