Your boiler is struggling to push water around your heating system because there is trapped air inside the internal pump, which is preventing your radiators from warming up properly.

DIY fix possible

Turn off the power supply to the boiler.Locate the pump screw on the front (a large flat-head screw) and carefully loosen it to bleed out trapped air until water trickles out.Retighten the screw, ensure your system pressure is topped up to 1.5 bar, and restart the boiler.