Aye Rog,
My wee place is an old cotters cottage with concrete floors in all rooms apart from the floorboards laid on half sawn larch bearers laid directly onto the ground in the living room. Walls are 2' thick rubble stone walls behind plasterboard with a 50mm air gap and no insulation between. They 'hoover' heat!
200mm insulation in the loft and make sure you keep it back from the eaves so that air can circulate above it. Put reflecting bubblewrap down the back of the radiators, so you don't heat the walls, valves on the radiators apart from the one in the bathroom, this ensures a cushty warm bathroom acts as a safety for the boiler and you can control which rooms get heated and which are kept aired. If you have concrete floors, try and insulate them and if you have drafty floorboards, hardboard does a good job of keeping the drafts out.
My woodburner gives off good radiant heat but also heats the fabric of the house [fireplace and stone chimmney] big heat sink, which then gives up this heat slowly through the house when the stove is out. Same applies I would suggest with your coal fire. Current gas boilers flu just vents to atmosphere and provides no heat sink with which to mop up 'exhaust' heat.
LPG very expensive and if under spec'd a disaster. Run a mains gas boiler of the correct capacity. Not sure about the combi boilers, if the power goes off your stuffed, with no hot water as it heats water 'on demand'. Whereas the ordinary condensing boiler can run at 90% efficiency [heat taken from 'exhaust' in condensation process] and heats water in a separate tank, so if a power cut occours, you still have hot water.
Other combinations of heating the water are numerous, but I suspect you're not looking for major upheavels.
See if SteveL comes on line, he has a plumber mate [#3 owner] and could probably give some good basic advise.
My regards, Bill.
