We have a Now half working mains stopcock, used to work fine, until we had a slight leak (round gland nut i think) so called plumber... he came. sorted leak but now when we want to turn off water it only turns off Half way then gets really stif until it wont move anymore and wont shut supply off.. Is there anything we are able to do to help it "move"? can get it to move bit more using grips, but not much more. thanks.