as the threads are plastic it is vital that the fittings are not cross threaded which will result in the seats not matching correctly. A gently hand pressure is needed and stop if there is any resistance before the parts are fully screwed together. Sometimes you may have to dismantle the unit from the cistern to discover the problem as they come together. replacement units are available in B&.Q. I would do it for you but I'm not based in England, sorry
Maximo
July 2008