IDEAL CLASSIC LXFF & FF BOILER?

We have this boiler controlled by a "Siemens RWB29" (and with a "Towerstat RSD" thermostat).

The controller is set to manual so we just press a button to get 1 hour of hot water and another button to get 1 hour of central heating. For some reason the central heating only works when I put it on at around 6am with the hot water. At all other times the central heatin will claim to be on (according to the controller screen) but the radiators won't heat up (hot water works fine at any hour of the day).

One possibility is that the controller is faulty and only activates the central heating at a certain time.. so I set the controller's internal clock to 6am and tried it. Another possibility is that the the central heating on this mode of boiler cannot operate independently of the hot water. I'm attempting to figure it all, but was wondering if anyone out there knows about this sort of thing?

Also, you know radiators... well the knob on the right-hand side controls the heat.. right? Whats the left-hand knob for? (I'm not refering to the screw pin for bleeding the air).

Many thanks for any help you can offer!
Adam