Your fan in the boiler is sticking.
We have had it a lot.
wd40 will fix it.
DO NOT let anyone try and tell you different. You'll need a corgi engineer out; but stand behind him and tell him that you are sure it is the fan. Then stand behind him and ask him to show you the fan.
It will tkae him 2 mins to take the covers off, the fan should turn by blowing on it, but it will be stuck. he can use his finger to loosen it, and then WD40 to help it on it's way.
Put it back together and that will be you - all sorted!!!
Good Luck
John
March 2007