I think you need to go for the inlet assembly.
http://www.showerdoc.com/shower-spares/triton/TRITO-S12721002-Triton-Inlet-Assembly-S12721002
If the Filter and ‘O’ ring assembly are the only parts that are faulty then I would just replace them.
http://www.showerdoc.com/shower-spares/triton/TRITO-83309100-Triton-Filter-And-O-Ring-(inlet-Assembly)-83309100
I note that the part number 83309100 is for the filter and ‘O’ ring not the inlet assembly