problem with potterton kingfisher II CF40?

Hi all, Hope someone can help, I have a pott kingfisher II CF40, floor standing, gravity fed i believe, as i cannot see a pump anywhere. situation is that with room stat on lets say 25 degrees and programmer for heating and hot water on 'twice' a day the radiators are not getting hot anymore even though hot water still seems to be going to and fro the hot water cylinder and heating that water.
The boiler does fire up but does for a short period of time, def not enough to heat any radiators. When the boiler shuts off i can hear a little bit of wat sounds like bubbling coming from the boiler.
If anyone can help with this problem it would be much appreciated.