Bristan mixer knob fell off and not mixing?

Have a Bristan shower which is 3yrs old (no receipt though)

The mixer knob has fallen off and looks like the plastic washer inside it has disintegrated - does anyone know what these are called and where can I get them from so I can put the knob back on?

Pictures here
[IMG]http://i53.tinypic.com/2rr17o1.jpg[/IMG]

Is there a way of identifying the model of the shower at all?
[IMG]http://i56.tinypic.com/oid7go.jpg[/IMG]

Also, the mixer hasn't been working properly for a while - sometimes its really good but most of the time not so much (using the basin taps gives you hot water but not the shower mixer)

Have read about the catridge but not sure how to get to it and how to replace it? Appreciate if anyone could advise or any recommendations for plumbers in London who can have a look?

Don't really want to replace the shower as its going to be really annoying with the tiling, not to mention the expense.