Thanks for all the comments, particularly the explanations of the grant system.
A bit more back ground.
House is actually 4 bedrooms and a box room as half a bedroom went as access to a self build extension. It is my parents house, they are both retired and my father is has severe arthritis...
We have an old Wilson boiler not a wall flame but the older wick type like an Aga oil conversion. This has performed reasonably reliably for nearly 50 years with yearly wick replacement. It heats part of a 5 bedroom house, by a single pipe gravity system with no controls, supplying 6 radiators...