Are you sure it was the stat that packed up? It does happen, but fairly rare. Chances are it is actually the switch in a valve or something like that. that would also explain why it doesn't work now despite timer and stat both being on.
Best approach is to first identify exactly which system you have i.e. s-plan, y-plan.
Then get an official diagram for it, you can download it from honeywell.
Next you check wire for wire that all is connected as it should be, and mark every wire you have checked.
In most cases, the wires/cables from the discrete parts to the centre are not marked/ wrong colour, and more often than not, it is not wired correctly.
NEVER use cpc (the bare wire in flat cable) or earth wires (yellow/green) for anything else than earth.
The wiring centre should either have a printed board with marked connections, or a 10 or 12 point double terminal strip. It is normal that not all points are used, that is done so every system can use the same set up, and, for instance, point 6 is always the same use.
If you don't have a multimeter, or you don't know exactly how to use it, don't do it yourself; you are working with 230 Volt, copper pipes and water. Not healthy if you do it wrong.
