The bleed valve on my radiator snapped off recently. I've drained the radiator down and am ready to replace the valve. However when I looked in B&Q they didn't have the right size. I then ordered one online which I thought was the correct size - I measured the thread diameter which is approx 6mm. However when I recieved the part today its too big - thread approx 11mm diameter. Did they send the wrong one or am I measuring it wrong? Thanks for the help!