Looking at
Enova.no which is the website for our energy-conservation efforts as well as an overview of different technologies and subsidies from the state (the front page is in English as well, click on the UK flag). They say a air-air heat pump lasts for 10-15 years, and removes about 20-60% of the energy needed for heating and warm water.
If you want I can always try to find some of the producers we've got here in Norway, they should last a lot more than 3-4 years. After all the producers MUST supply a 5 year warranty by state-law so 3-4 years would be VERY bad choice for the producers...
Cobos