Sorry, should have mentioned that I have repeatedly vented the air out of the pipe work next to cylinder on many occations but still seems to be generating air and still sounding like there is sloshing/bubbles going through cylinder and also can be heard back to gas boiler as well.
When using just the gas boiler to generate hot water in cylinder lots of bubbles/sloshing can be heard. Seems to cycle i.e. sloshing for 10 secs then no noise. Central heating does not generate any noise on its own.
Any ideas - perhaps debris biuld up in cylinder?
Pump is a Grundfos 10/60...