At £21,000 to install mains gas, this option sounds like a non-starter. But you could consider liquid petroleum gas (LPG) instead. The tanks can generally be installed underground (subject to strict siting criteria) for around £750 plus the cost of the hole digging and service pipe trenching. This would be subject to a two-year minimum contract with the gas supplier, but the additional cost of LPG over mains will be more than offset by the capital cost savings. Using oil for the boiler and range cooker (and/or an LPG bottle for a gas hob) is another option. If you do have a ground source heat pump, any such installation should also have a secondary system installed.The trouble with an electrical secondary system is that in the event of an electrical failure both systems will be down and you will not have heating or hot water, or a means of boiling water. Bear in mind that with the two open fires, your options are now restricted by Part L of the Building Regulations and the need for air leakage testing and Standard Assessment Procedure (energy rating) certificates. You will need to drastically improve insulation elsewhere to compensate for both the fires. If you use an enclosed log-burning fire with a 200mm flue or a jet master-type fire with a larger flue you can gain better efficiencies which will help with the energy rating calculations.