Replacing a 15-year old boiler with a modern condensing boiler should reduce your fuel bills by one third. If you also install good heating controls, the savings could be around 40%. A new boiler would also reduce your carbon emissions - and because of the monetary savings, it will be one of the most cost-effective ways of cutting a big chunk off you household's emissions.
Before getting a new boiler, make sure you've taken all possible insulation and draught-proofing measures. The boiler can then be sized to your new, reduced, heating demand. This is important, as if a boiler is too big (over-sized) it will not operate efficiently most of the time. Domestic gas and oil boilers are covered by an energy label scheme - look for boilers that have an A-rating, so meet the highest efficiency standards. You can also look for 'Energy Saving Recommended' logos on the boiler and on associated heating controls. These are awarded by the Energy Saving Trust. A long warranty gives an indication that the manufacturer has faith in their product.
Only condensing boilers will get an A-rating: they increase efficiency by recovering heat that is normally wasted in the hot flue gases given off by a conventional boiler. In the early days of the technology (over 20 years ago), condensing boilers did have some teething problems, and this impression has persisted. But the technology is now well developed, and modern units are as reliable as any other new boiler. One initial problem was with the type of heat exchanger used. If flue gases condense within a ferrous metal heat exchanger they can cause corrosion. New boilers should have a heat exchanger made of a metal resistant to rust such as stainless steel. Getting your pipes and radiators flushed out (to remove built-up gunge) will cost a few hundred pounds, and should be done while fitting any replacement boiler. Annual servicing will ensure that the boiler keeps operating efficiently.
All modern boilers are complex pieces of equipment and it is worth spending time choosing the right one for you. Using a good installer will help: if an installer does not think much of condensing boilers, then it would probably be pointless asking them to install one - they are unlikely to put in place the heating controls that will get the most from the system. Look for someone who is knowledgeable about the technology and confident that they can give you an efficient and convenient heating system.
If unable to get a personal recommendation from a friend (often the best way), then you could look for a tradesperson who belongs to an association that aims to maintina high standards. For example, the Institute of Domestic Heating and Environmental Engineers, the Chartered Institute of Plumbing and Heating Engineering, or the Association of Plumbing & Heating Contractors.