The PRD is located in the bottom of the heating can behind the outlet pipe and is held in place by the outlet pipe. It is comparatively easily replaced, by removing the 2 screws holding the pipe inplace pulling the pipe forward and down will reveal the PRD. Fitting is obvious but lubricate seals and ease parts into place, do not force.
The PRD can be ordered online by clicking on the link below.

http://www.showerdoc.com/shower-spares/triton/TRITO-83301330-PRD-(Pressure-Relief-Device)-and-'O'-ring-(Pair)