Boiler without a thermostat

I moved into a new flat on Saturday. It has a Vaillant Ecotec pro 24 boiler. I know this is working as I'm getting plenty of hot water. But the radiators aren't coming on. Looking at the manual, there's supposed to be a radiator symbol that appears on the display when the radiators are being fed, and I never see that. The system has the standard Vaillant timer panel on the front. I've got it set to "on". As far as I can see, there's no thermostat attached to the boiler. So I'm not sure how the system knows when to start feeding the radiators. Any advice would be most welcome (as it's starting to get be bit chilly!)