Older Oil and Gas Boilers have efficiencies of about 85%.

Newer condensing boilers have adjusted efficiencies in the range of 92% to 96%. In effect you save about 10% of you heating bill due to efficiency gains.

A condensing boiler is designed in such a way to recover the heat energy which is normally lost to the atmosphere through the flue. By recovering this energy, the boiler becomes more efficient.

A reputable heating engineer can replace an existing boiler with a new condensing boiler relatively easily and the savings should pay off the cost in a few years.

Oil vs. Gas:

Oil is more expensive than gas and it is also by far a more polluting fuel. If you install a condensing gas boiler, the extra cost installing gas to the house will be offset by the reduction in annual heating cost. Also, your carbon footprint will be greatly reduced.