If you live in an upstairs flat, it won't be outside the building. Most likely there isn't one if you can't find it. In which case you drain the system by unscrewing one of the ens of the rads - the lowest one to the ground if the's any difference. Great care needed and lots of newspaper to avoid excess water getting on carpets etc. A few gallons will come out. You can control the flow by screwing the thing up again as you go. Empty plastic ice-cream containers make a good receptacle for catching the fluid.