There could be a few reasons why your radiator is cold at the top. It’s possible that there is air trapped in the system, which is preventing the hot water from circulating properly. Another possibility is that the radiator isn’t big enough for the size of the room, or that it’s not getting enough heat from the boiler. It’s also possible that there is a problem with the thermostatic radiator valve or TRV, which is responsible for regulating the flow of hot water to the radiator.
If you are in Liverpool and need a plumber to help diagnose the issue, you can try searching online for plumbers in the area, or ask friends or family for recommendations. It’s important to hire a qualified and experienced professional to help fix the problem, as attempting to repair a radiator can be dangerous if you are not trained in this type of work.