I have hot water but no heating - any ideas?

[MrsPear]

So I have a Worcester combi boiler, Honeywell timer clock underneath and a thermostat in the hall way. Boiler fires and produces hot water. Timer set to continuous.
Thermostat has been turned up to try and get the heating to come on - nothing.
What am I doing wrong?

Valve broken probably you will need an engineer

Oh lord just as we are waiting to see if the landlord is going to let us stay another year! I might just call someone out myself and see what they think first. I was hoping someone would tell I'd forgotten to switch a magic button!

To explain a bit more there is a valve that diverts the water from the heating to the hot water it gets stuck. It's not difficult to fix but needs an engineer unless your very handy.