On Friday, 16 February 2018 19:52:51 UTC, Murmansk wrote:

I need to get under the cistern to replace the ballcock as it's mounted on the bottom of the cistern not the side ie the water comes in underneath

no no no. You just undo the big blue plastic nut in the photo, and the ball & arm come away. Replace the rubber washer/diaphragm with the right type, there are several, and put the ball/arm back in place & refit the nut. Then just check the water level it fills to is ok.

I assume you know to turn the water off first


NT