i would throw it in with hwc - nothing worse than having to crawl through a ceiling space or under a floor to find and adjust a prv

gives better accessibility and better protection from freezing also 

you can still do what you want to do depending on the existing plumbing



normally in my experience the high pressure feed to the ceiling tank and low pressure hwc feed went up/down through the hwc cupboard - if this is the case then:


- bridge the low pressure feed to the hwc and low presure feed to shower in the ceiling space
- use the old high pressure feeding ceiling tank that goes up inside hwc cupboard to supply 3in1 & prv to hwc inlet
- take a tee off after prv and connect to old low pressure feed to hwc which is now equal low pressure supply to shwr

should not have to touch hot supply unless installing temp valve then additional tee required (also after prv)

of course this all depends if pipework travels up/down hwc cupboard!!

Goodluck!!

hey, no its not a closed loop wetback, there are uncommon here.