It’s your landlord’s responsibility to fix it. You should report it to them in writing, and expect a response within 24-48 hours if you have no heating/hot water and it is winter. If it is the warmer months, you can expect a response within 5 days, unless you have no hot water at all. Only Gas Safe engineers are legally allowed to carry out repairs to gas appliance so check that anyone sent to repair it are on this list.
You could be entitled to claim compensation if fixing it is delayed for an unreasonable amount of time for any significant inconvenience or cost (like having to pay for alternative heating).
If you’ve talked to your landlord and they still haven’t fixed the problem or acknowledged your complaint check to see if they are on Unipol’s Code of Standards here so you can put in a complaint to Unipol here.
If they are not code or you need some additional help just contact LUU Advice.
It’s worth bearing in mind that if you rent through an agency, both your landlord and the agency are equally responsible for making sure repairs happen. The agency can’t refuse to do essential repairs by saying that the landlord won’t authorise it.