either 22mm or 15mm with do the job perfectly. what size of your cylinder secondary return pipe? 15 or 22?
isolate the power mains, open the junction box, and take a picture. then I will have an idea of how to replace it with nest
you need Gas Safe registered person to do the work. remove and put it back. anything between £200-£400 is reasonable.
by the way, you don't need...
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