If it’s a radiator that’s leaking isolate it by shutting the valves at either end of the radiator.
To do this turn them clockwise, if the valve has a locksheild cap on it this will need to be removed so you can get to the valve, the heads of thermostatic valves should also be removed. Place a bowl to catch the drips. If it’s the pipe work that’s leaking you may well not know until you spot a wet patch on the ceiling.
If your confident that you can drain the system then do it, otherwise its best to call us on 07730 560422 sooner rather than later!