903.33 is the correct part number. The cartridges may require replacement after 9 years but it's worth checking the inlet filters on the cartridges first to ensure it's not a blockage that's causing the problem. If water is getting to both sides of the shower and the filters are clear, it's likely they'll require repalcing. Mira can come out and do this for you including parts for a fixed price. Ring them on 0844 5715000.
Bob The Builder
February 2011