which part do i require?

You will need to replace the pilot valve.

Unfortunately the pilot valve for this shower is obsolete, however...........

This is the Pilot valve for the Sport which is still available

http://www.showerdoc.com/shower-spares/mira/MIRA-419-87-Mira-Pilot-Valve-419-87

You can adapt it for the Essentials, you need to swap the white spindle that carries the knob with the one on your old valve.

Check where the notches are on the sport spindle, because when you refit the essentials spindle it is important that you fit it in exactly the same position as the one you have removed.

The pilot valve is a common fault

Turn off water and power

Put something in the waste just encase you drop a screw!

Remove cover

Remove splash plate at pipe entry

Remove 4 screws from 'U' clamps and 'U' clamps

Later models have “Torx” screws so you will need a “Torx” screwdriver with a T10 Bit.

Remove screw above letter 'A' on drawing http://www.showerdoc.com/shower-spares/mira/MIRA-PARENT-16-Mira-Sport---post-1992

Ease pilot valve forward (if there is no movement on the pipe work you will have to disconnect it at the inlet connection take care to hold the brass body with pliers when slackening the nut)

Swap the wires over from one pilot valve to the other one by one

Then reassemble

Turn on water and allow the water to come thru unit before switching on power.