On 16/02/2018 19:47, Dave W wrote:
On 16/02/2018 19:42, Murmansk wrote:
Fair enough but the thing is kind of "built-in" so getting to the
underneath of the cistern could be a serious challenge!

You need to replace the ballcock, not get underneath the cistern.

The OP's "ballcock" is a unit that includes the filling aspect that
attaches to the bottom.

Different internals will not generally be compatible.