Unless your home is very new, you’re probably losing heat through draughts around doors and windows, gaps around the floor, up a chimney or two, and a whole host of other little holes and cracks in the external envelope of your home.
Bunging up these gaps and cracks is relatively inexpensive, but could save £100 a year or more, and make your home feel much more comfortable besides.
Draught-proof your doors and windows using seals and excluders available from your local DIY shop, seal your skirting boards with a silicone sealant, cover letter boxes and keyholes, and fit chimney draught excluders, balloons, or pillows (black bags filled with screwed up newspaper work well) – but remember that they’re up there if you light a fire!
Particularly if you have sash windows, you may like to consider an overhaul of your windows by a professional. They will insert draught-proofing strips into the sides of the window sashes – this can cost up to £300.00 per m2.
Remember though, your draughty single-glazed windows are well-ventilated, and draught proofing may result in a marked increase in condensation.
After plugging up your gaps (leave all intentional ventilation open), you may find that your draught proofing efforts are so successful that you will have to introduce managed ventilation - although this is very unlikely in an old house. 
NB: Always ensure gas appliances have adequate ventilation – check with your local gas technician if you are unsure.
AND if you do use chimney draught excluders, balloons or pillows, you should take them out when you’re not heating the house (from spring til autumn say) in order to properly ventilate the chimney stack. If you don’t, you may find dampness and mould forming within the chimney stack or on the chimney breast.
© green.co.uk 2020