Hi All, I have a leaking isolation valve just under a cistern, the copper pipe entering the isolation valve from underneath is 15mm, the plastic thread coming down from the cistern measures 20mm across the thread, the only isolation valves I can see are 15mm to 15mm or 15mm to 22mm, am I measuring incorrectly

Regards to All and Thanks for your help in advance

Are you measuring correctly ? Are you reading the sizes correctly ? O.D. vs I.D ?
As far as I know, you can purchase just about any conversion fitting. Please, measure again.

Back to you. Milo

Hi Tim and welcome to The Plumbing Page at ANHD.com. Are you in the UK? By "isolating valve" are we talking about the cisterns shut off valve? Your measures sound correct until I get to the 20mm, (3/4 inch ID) If you have a 15mm (1/2" ID in IPS) the you either have a reducer in the tank or you made the wrong measure. Son what's your problem? Either repair the leaky valve or replace with one the same size. Good luck, Tom .