You can carefully separate the front shell joint with a flat bladed screwdriver. Insert the screwdriver in the joint between the two grey plastic sections of shell and work around carefully levering them apart. They are bonded in four places but with care you will be able to separate them.
There is no need to remove the yellow section of shell.
Once inside check the electrical connections for rust because they are prone to this due to water ingress. If any connections are rusty, clean them with a small wire brush. There is a spade terminal that can be unplugged, cleaned and re-connected. This is a cause of issues.
Once cleaned, re-insert the batteries and test the unit by switching between on and off. If the valve opens and closes (look down through the water inlet and outlet to see) then it is fixed.
If any of the gears are damaged or the motor does not turn it may have been damaged by frost in the recent cold winter. This is not repairable.
When you have cleaned the inside of rust and if all is well you can re-assemble the shell. You will need to re-bond the joints between the shell sections with some superglue or, better still, polystyrene cement.
I did this with mine and it now works fine.
I hope this helps
Nigel Witham
April 2011