Ground Source Heat pump - open loop system.
It works well for us and does all our heating and hot water. Had very bad advice from various sources at time of insulation. So had lots of problems in the first couple of years, but system now working well. Our MCS accredited installer subsequently was stripped of his MCS accreditation and has gone bust! and the company making our heat pump (TES ltd) also went bust! Very difficult to get non-biassed information from anyone at the time we were installing in 2010. Now trying to apply for RHI.