Thanks footprints and scatmanjohn, I have discovered that the system was using a Danfoss 103 which is single channel so have replaced it with a Honeywell ST699. Fingers crossed I should be able to sort the problem.
I'm helping out with a heating system which hasn't been used for a while (empty properrty). Hot water works, but no Central Heating. The system is Y plan with timer controlling both HW and CH and stats controlling HW and CH independently. I changed the circulating pump (original leaking and...