Siemens RDJ10 - Help Please!!!!!?

I have just moved into a new property which has a Siemens RDJ10 Thermostat installed. I have a combi boiler installed in the loft (a bungalow). My problem is that when the heating comes on say in the morning it starts on say 15c but takes ages (hours) to get up to the max set temperature of 23c. I feel like i'm having to set the heating to come on stupidly early in the morning so when I get up it's at least a bit warm. Same for when I get home form work on an evening. Does anyone know how I can get it to start at a higher temperature when it comes on? I'm freezing!!

Thanks