Okay mate, thanks. I'll try the air bleeding and let you know how I get on. Cheers
Your boiler measures the temp of the return water back from your radiator circuit, and uses this to determine if it should keep firing.
If there are restrictions in this circuit (due to air/ sediment/ balancing - or any way that the path can short circuit) the boiler will cycle to try and maintain a baseline temp between flow and return. It's like Cruise Control for your central heating.