honeywell st6400

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...

I am attempting to replace my Central heating and hot water programmer (Honeywell st6400c) with the tado extension kit, I already have a tado smart thermostat installed on a hard wired room thermostat. When looking at the junction box wiring beneath the programmer it didn't seem to match any...