Low flow

It makes more sense to replace complete flow valve .

If the coil has gone then the flow valve will not last much longer and the valve comes complete with a new coil and switch.

http://www.showerdoc.com/shower-spares/mira/MIRA-1563-508-Flow-Valve-Assembly-10-8-kW

Sport Flow Valve

Turn off water and power.

It may be useful to take photos before and as you strip down the unit they can be invaluable when reassembling

You will need a 'Torx' screwdriver with a T10 bit

Cover the waste pipe unless you want to loose the screws

Remove screws from 'U' clamp, switching assembly (at top of flow valve),the clip that holds flow valve and heating tank together and the screws at heating tank outlet pipe.

Ease tank and valve forward and slightly right, pull the heating tank away from the valve some water will run out of the tank at this point.

Allow the tank to hang on the wires strip off the wires from the existing flow valve and refit them to the new valve.

Fit the new check valve (supplied) to the flow valve outlet (Large end in)

Using the new 'O' rings (make sure they don’t fall off or become misplaced when fitting) fit the new flow valve make sure that it locates correctly with the heating tank clip and the locating pins at the top of the valve locate correctly with the switching assembly, as you push the whole assembly back into position ensure that the heating tank outlet pipe is in position and the inlet connector is in place.

The above procedure is not technically difficult however three pairs of hands and the ability to levitate will help and for sure you will be glad when you see it all back in place!

Once everything is back in place refit all the screws making sure that everything is located correctly and nothing is misaligned.

Turn on water and examine for leaks switch on power then turn shower on at cold setting and run for 1min. Take care when doing this as shower is now live.

Refit cover and test at different power settings.

If every thing is OK have a shower you deserve it!!!!!