my central heating boiler runs but doesnt heat the radiators?

the problem is as follows, when i swithch on my boilerthe pump runs (not sticking) and it ignites heats up then when at temperature switches off then on every thirty seconds or so as it hetas and cools,but the water doesnt circulate heating the radiators and it sounds as if there is air in the boiler.my radiator has the old type of bleed on the bottom inlet / outlet pipe any advice on how to bleed the system would be greatly appreciated