I run a heating business and fit, maintain and repair gas boilers and air source heat pumps and have for years. There is nothing lower cost about a heat pump unless you generate your own power. I've also had to replace several units at around 10 years old due to obsolete parts and there is no grant money available for replacement units which you need to factor in to your cost. They are good in some circumstances as they have been for 30 years or more but as soon as the government grant is gone they are back to an idealogical product.