how do i change the pump or whatever its called in my cistern?

The pump is called a syphon.
High level and low level:
turn off the water bale the water out of the cistern.
Under the cistern:
Undo the nut on the cistern to pan flush tube.
Undo the nut holding the syphon in the cistern pull it up and out of the cistern reverse procedure to fit new one.

Close coupled;
that's the one with the cistern fixed to the pan.

Turn off the water bale out the cistern
Undo the water feed joint at the inlet valve pull pipe away from fitting/valve.
Remove the top wall screws holding the cistern to the wall [if fitted]
Undo the cistern to pan fixing bolts lift cistern clear.
undo the syphon nut on bottom of cistern remove the syphon. Reverse ins fitting new one.
With new syphon get a new pan to cistern fitting kit comprise of metal plate large doughnut shaped cistern to pan seal two bolts and rubber sealing washers. If you still don't know what to do get a plumber.

Nobby
February 2009