how do I change cartridge on a shower diverter valve

Hi,

I have a showier diverter valve on the wall for the shower. It ha 2 chrome dials - the top one is one and off and is a selector for either the showier hose or the fixed ceiling shower head. The dial below rotates for hot or cold.

The top dial is now spinning freely - albeit i can just about turn the shower on by putting some force and turning the dial till i feel it resist.

Any idea how I fix this?
Is there a cartridge inside I need to replace?

I tried undoing the the head of the chrome valve but i can find any grommet or small screw to prise it open.

Thinking of giving it a try to save money if it's easy enough otherwise will have to get someone in.

And the make / model / photograph of the shower?

No idea on the model. Here is a photo. ( I unscrewed the lever to rotate the top knob thinking that would help twist it off but not much help)

1. The knob may just pull off.
2. There may be a grub screw in the side of the knob holding it to the spindle.
3. The lever in the knob (would have been at 10 o'clock in your photograph) may unscrew to reveal a grub screw underneath it.
Someone must have fitted the knob after putting the rectangular cover plate in position, so there must be a way of removing it.

Tried 2 and 3 no grub screw there. Trying to pull the knob off, not successful yet, may try prising off with a screwdriver to get some leverage.

I noticed if i pull the knob then turn i can get some resistance and get the shower to turn on and off..

What could have happened behind the knob? Is it just slipping somewhere on a spindle or something like that?

1. There is most likely a female spline within the knob, and a matching brass male spline on the spindle.
2. Either the male or female spline (or both) are probably worn, so the knob just revolves on the spindle.
3. By pulling on the knob, you are making the splines engage at a different, and less worn, portion, so there is grip again.
4. Are you absolutely certain there is no grub screw, likely to be M2 or M2.5 with an Allen socket head, under the removed lever?
5. Have a look and see how the other knob comes off. Likely to be the same, but no guarantees!