Not a boiler fault...
Check up in your airing cupboard to see if you have a motorised valve; possibly that is the issue... Normally silver (Honeywell), white/blue (ACL/Drayton/Sunvic) or Red (Tower), with some models there is a manual override switch to get you temporarily going, in most cases a plumber with experience in draining heating systems will be required.
If you are gravity - Time to replace the boiler unfortunately as many of these system gunk up over time and it's cost in-effective to do the rectification work.
-> Thermgas.co.uk
ThermGas
December 2006