Replacement
If your boiler was installed prior to 2007 then is it almost certainly a non condensing type with an efficiency rating of between 75-85%, when oil was available at 25-35p/ltr this was considered an acceptable level of efficiency but with oil now at 60-65p/ltr and the introduction of condensing boilers that deliver efficiencies in excess of 95% it may be time to consider upgrading both your boiler and heating system.
15-20% savings on your current oil use are achievable and represent a very real return on your investment, better than any bank account.
Contact me for further details.