A heat pump is an electrically powered device that transfers heat from one place to another, through a heat exchanger, which concentrates the heat to enable it to be usefully used.

A ground source heat pump consists of pipes in a trench or bolehole which bring underground heat at 10-12^oC into the building.

Grants are sometimes available for installation, which can be expensive (typically £6,000-£12,000). However, running costs are low – just the electricity to power the pump (which could be renewable or self-generated electricity). You can expect to save a considerable proportion of your heating bill – some estimates say up to £800 a year (which seems optimistic to me).

More information from the Heat Pump Association.