I had to fit a new syphon and reconnect the ball valve. A very small very slow drip persists (not more than wetness on the pipe), so I'm going to replace the bottom entry ball valve and the isolator/15mm copper pipe feeding it. The standard size of the plastic threaded end of the bottom entry ball valve appears to be 20mm approx. with a 23mm nut (outer dimensions). So that would require a 3/4" connector that end? There is 420mm horizontally between the vertical axis of the connection to the cistern and the far end of the horizontal isolator, approx. 1 inch vertically. I'd therefore need a 500mm 15mm flexible connector with in-built isolator?
Assuming all these dimensions make sense, would this ball valve and this connector fit together?
Get a 500mm x 15mm flexible tap connector with a service valve. Personally I prefer having a separate isolation valve as if it ever starts to leak you only need replace the valve which is cheaper than a flexible tap conn that incorporates a service valve.
The flexible tap connector will come with a compression fitting on one end which will go straight onto 15mm pipe and the other end is a 1/2" female fitting that will go straight onto the plastic thread of the bottom entry ball valve. Fit a service (isolation) valve before the flexible tap conn.