help! glow worm betacom 24c no heating?

Ok here goes... the above boiler was working fine!

After coming back from a weekend trip i noticed the heating side of the boiler is non operational. The hot water is fine and hot. The pump is spinning, the pressure std 1.5 bar. No flashing led's, lights up to 80c. The CH flow pipe is hot but the return stone cold. No room stat attached. Anyone have any idea's? I'm thinking the diverter valve, but unit only 24mths old.