Identify how the handle is fixed to the splines of the iBox. There are two different ways that this can be fixed, If there is a hole on the handle, then there is a grub screw which you simply loosen and the handle will come off. If there is no hole, then it is on a quick click connector. It is always good practice to remove the handle at the same place that you want to put it back on when you have finished the service.
To remove the thermostat, first you must remove the safety stop sleeve. It is a good idea to remember the orientation of the safety stop point. To remove the flow control cartridge you must unscrew the sleeve by loosening the back nut that holds the cartridge in place. The whole cartridge will come out in one barrel, if you ever need to replace the cartridge unscrew the extension spline off the old one and install it onto the new one using an allen key.
On the back of the cartridge are to locating pins which marry up to the two holes inside the brass body, so that it is in its correct orientation. Make sure the pins are located correctly, then close the back nut on and screw in tightly. To gain access to the non-return valves you first need to remove the faceplate housing ring it is held in by 4 nylon screws. Remove the screws and pull off the housing ring. Then remove the brass bock by unscrewing the bolts giving you access to the non-return valves.
You can check the operation of the non-return valve and remove any debris by pressing the plungers. To replace a non-return valve, use a small flat headed screwdriver and put it just inside and to the side wall, you can edge a non-return valve out and to replace it simply press it back into its housing. Before installing the brass block back make sure that all of the O-rings are in place, then screw all 4 screws tightly. With the securing ring use silicone grease on the Nylon threads then tighten the screws being careful not to cross thread. When tightening them you just want to gently nip the securing to the tiles.
To reinstall the finish set, face plate, handles and to set the thermostat temperature first you need to screw on the decorative shroud that simply goes over the flow control and screws into position. Then the safety stop sleeve, be mindful of the orientation that this goes onto the thermostat housing. On this particular model the safety stop point is at the bottom at 6 O'clock, and the button is on the underside of the handle so this safety stop point goes at 6 o'clock. Sometimes the safety stop point on the handle is at the top so the safety stop sleeve has to go at 12 o'clock so that the button corresponds with this part. Lightly Grease the O-rings of the sleeve and press onto the housing and against the tiles. The flow control handle has got a mark at the top in the middle to mark the off position, so the marking goes at 12 o'clock. This is on a quick clip connector so you will hear a clip when it's pushed on.
Then set the temperature of the shower.