Change the boiler. The advance in energy efficiency over the last 20 years has been huge.We have an old 20yr boiler on kerosene with rads which are not very effective. We have an electric shower. But we have no on demand hot water at sink kitchen/bathroom when needed. As there are only 3 in the house oil is not looking at being very efficent. Looking at alternatives that can make life easier, ie. undersink on demand water heater kitch/bathroom with panel heaters in rooms that are actually being used. These can be un plugged and moved to any room as needed.
Solid fuel non boiler in kitchen.
I changed a 14 year old boiler that was located about 10 metres from the house to a new Grant boiler with heat/water & zone controls and oil usage literally halved - and that not an exaggeration. A single 1000 lt fill will do me all winter and its a good sized house.
Total cost was about €3k and we'll have saved that in oil by end of this winter
Have a central heating service person come out and advise