Had a look at pic, You need a New Diverter Valve i'm afraid, the hole is for one of three clamping screws for the diaphragm housing You should be aboe to obtain a SHORT DIVERTER VALVE (No Diaphragm housing ) use the old one again, but the difference in cost is very little so may be easier to obtain whole thing
www.elanar.co.uk
September 2009