There is no 'off the shelf' answer to this one. Its like saying 'why won't my car tick over nicely."
One thing is for sure your boiler is not achieving a clean burn and an approved oil boiler service is your responsible course of action.
If you had it serviced before the winter period your engineer should be called back. Modern oil boilers do not need mid winter services to see them through the cold period
cledlyn, January 2009
