This is a tough question, as it mostly depends on your rental agreement. In the UK, most rental agreements will include a clause about repairs and maintenance. Commonly, the landlord of the property would be responsible for repairs on fixtures like heating systems and boilers.
When my boiler needed replacing, my landlord advised me that he would pay for the replacement but he asked if I could organise it. It was pretty simple to do, I just called a plumbing and heating company offering boiler installation in fife and they came out to my home and replaced the boiler a few days later. I think by law the landlord really should have made these arrangements but I was happy to do it as I just wanted the issue dealing with quickly. If you’re unsure about your current situation, you can always check your contracts or contact the estate agent that let the property to you.