Maybe time to look for a more modern unit ?

I grew up in old, draughty, uninsulated houses!
Top tips -
Blu-tac tin foil behind radiators to stop the heat being absorbed into the walls.

Insulated curtains over windows and doors help keep out draughts and prevent heat being lost. Draught excluders are good for this too.

If possible, heat just one room and keep the door closed. Uses less energy than trying to heat the whole place.

Carpets, rugs, mats, etc help to insulate the floor and keep the chill out. You can stick flattened cardboard boxes underneath to add insulation.

Wrap up warm!!! 🙂

Thanks for the suggestions :)

Randomlygenerated: We have done half of that, but will try the others :)

DaveInver: It's actually a council flat and they won't do anything about it (I have had to call them out to fix the boiler a number of times, when I asked for a replacement thinking it will be cheaper than having to come out so often to fix the broken one, they said if they do it to mine they have to do it to everyone's which isn't in the budget)

Cryptocurrency mining rigs can often make you a profit while simultaneously heating up the room. Takes an initial investment though, and you have to know what you're doing. I never did it to produce heat, as I live in a well insulated place - it actually gets way too warm with it running!

Hang blankets along the walls. Leave a distance of at least a foot between the blanket and the wall. It will cut down on the area to heat and increase your insulation factor. Just make sure your heater is inside the blankets where you are.

You could perhaps ask the landlords or council if they offer any grants/or help regarding insulation, renew heaters etc. Even if you own the flat there may be help to be had.

Blankets everywhere...regular and electric. Thick, warm socks. Hot teas and soups.

Get an electric blanket

Boil water on the stove