HOW DO I REPLACE THE PRD ON MY TRITON OPAL SHOWER?

I have a seven year old Triton Opal shower. It has never been brilliant and has seemed to run hot and cold during use. We haven't used it for some years and I've just had a go at 'fixing it'. As soon as I turned it on, the overflow pipe at the back shot out water. On reading the User Guide (yes, I still have it!), it appears the Pressure Relief Device has operated. It says that it must be replaced (not just reset). There is a picture of the part in the User Guide but, having openned the front of the unit, I can't see it! I presume it is inside or behind something else? Can anyone tell me how to find it and how easy it is to replace?