Hi
I recently had the same problem.  We checked the rads for air.  We then drained down the system but had to call out a plumber to repressurise the system.  This cost a total of £47 plus vat.  That is why i am now searching the web, this happened 10 months ago and my sister-in-law sorted it out in 10 minutes! So ideally I would like to learn how to save myself that money.  So basically its a case of checking the rads for air, draining down the system and then repressurising your boiler.  I hope this helps.
                
 TJ
                
 November 2008