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Thank you both, with your help and this great blog post: https://www.urbanplumbers.co.uk/how-to-install-nest-thermostat-on-a-y-plan-heating-system-uk/ I got everything working! The wiring centre was directly below the programmer. And I've replaced that mystery earth wire!

I'm trying to replace an ancient Honeywell ST6400C programmer which controls our hot water and heating with a Nest Thermostat Heatlink. When I opened up the Honeywell, I was met with this wiring, which puzzles me greatly - it appears that there's only one wire leading to the central heating, but...