Wet patches on internal wall and loose plaster near radiator pipes

Hi, Thanks for the question. It sounds to me like the radiator has been knocked a few times hence the crumbling plaster, however if it was a leak in the plumbing I would expect more symptoms such as loss of pressure within the central heating. My guess is this is not related to the wet wall but I wouldn't rule it out without a thorough inspection first. Water finds its own route through walls and ceilings, the fact that the damp area is near a window would be my first point to have a closer look. I would be looking for cracks and issues with the guttering (Can you see light between the gutter and the wall? If so water is running off the roof and straight down the wall). I hope this helps! All the best, Justin