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Connected as above then programmed the stat and it works perfectly. I expect if a MoMo valve is fitted again then the wire to old 4 would go to new C. Thank you everso much. :D

Thank you for the quick response. Originally the system had Sunvic MOMO valves on both zones and HW. Now I have the common Honeywell zone valves which have the open lever, synchronous motors and spring shut. I can't see the model nos and likewise with the thermostat as it has worn off with age!

My old stat is a late 1970s Honeywell which controls a 2 port Zone valve as I have a 2 zone heating system and hot water. There are 2 pairs of red/black wires connected to the 4 terminals numbered 2, 1, 4, 3. The first pair are black on 2, red on 1 and the second pair black on 4 red on 3...