Hello,
It's pretty easy. You buy the PRD already with the o'ring, and just push it into the bottom of the heater tank. We can supply this part for you. The part number is 83301330. You can call us on 0845 126 4141 and we can supply this for you, or visit our web site - this is the actual page for the Triton T80i
http://www.showerspares.com/products/spare_parts/triton_t80i_electric_shower
NATIONAL SHOWER SPARES
July 2008