I bought one of these from Screwfix http://www.screwfix.com/p/self-cutting-tap-15mm-x/21250 to connect to my central heating pipework since I don't have a drain off valve. I've connected it up, but it's leaking very slightly and it seems to be from the locking nut. I've done it up very tight, but not too tight since I don't want to crush the pipe. Is it faulty? How do I stop the leak?