Potterton puma 80, CH not working?

Hi all i hope i posted this in the right place,

Basically moved into my new house last month and the house is fitted with a potterton puma 80 combi boiler. The boiler has worked fine for a month and then today decided to stop working, The hot water is still working fine but the radiators arnt getting hot? underneath the boiler the copper pipes are all labelled up the CH flow is hot but the CH return isnt.

Im a bit stuck because im out of my depth here. Anybody had the problem before? I dont really know what to check other than the pressure which is 1.5Bar

Thanks, Adam