Do you mean a wall hung thermostat? The only thermostat I have is one on
one of my rads (in my lounge). I don't think I have some master
thermostat.

Well, spend your money having one installed, rather than fixing the clock.
It will not only make the timing work, but will save you a fortune in gas,
as currently the boiler will be on for the entire time that heating is
selected on, not just when the house is cold. As it appears you don't even
have widespread TRVs, the house will likely overheat, too, causing really
quite appalling energy efficiency.

Christian.