Well, reading back my last post I wasn't really all that clear!!
You don't say whether you're using oil or gas, but the chances are, these cost less to run than a heat pump will for your current needs. Heat pumps as a retro-fit generally make more sense if included with a whole package of energy improvements that significantly reduce your overall heating needs.
Most heat pumps run at a lower temperature than a traditional central heating boiler, so adjustments are often required to the rest of the system (changing the rads for instance). I have read of a few companies who are making high-temperature heat pumps that are a better option for replacement of a boiler, but I've no idea what these cost or how widely available they are here at this point.