Alpha CB24 Combi Boiler not heating up radiators?

My home radiators are not heating up enough to warm the flat, the Boiler works fine, had the radiators cleaned out yet still not enough heat emitting from them to warm the flat.

There is no pressure viewable on the gauge other than the standard pressure for the boiler. I may be missing something and dont want any more unncessary costs for someone to come out, any advice folks?

Nick