Never used radweld, always thought that it was just delaying the inevitable and tried to fix the problem properly if I can.... Try having a look with small torch, the light can sometimes show a glisten of water where the leak is. There's a rad on ebay at the mo (item 4598536205), not sure if it will fit a valver tho, else try gsf or a breakers. I'd check all the pipework thoroughly first myself, and get under the car if you can with it running for a good look if it helps...