Ideal Icos Boiler, Heatrae Sadia CL170, Reliance Pressure gauge?

My CH/HW system consists of the above components plus a room thermostat and control panel beside the boiler. At the moment I have Hot Water but none of the radiators are heating up. There is a constant noise from the location of the Heatrae and pressure cauge which I think is coming from the pump. Also, on the pressure gauge there are 2 arrows, the red one is at 1.5 but the black one is sitting on 0. I have no idea about CH/plumbing etc in general so don't really know where to start. Any thoughts assistance would be appreciated.