Did some more digging, these where marketed by WDP but are a rebranded Pure Energy reg. We all learned something !
Once you take the bonnet off there are only like 2 - 3 orings if I recall, 1 to seal around the pin, two on the piston. They where likely urethane orings, just clean of any rubbish, lube with dow33 and reassemble, if the orings are perished (unlikely) you could substitute buna 90 duro, 70s would likely leak under that amount of pressure. The only other sealing surface was a reg seat but that is a pressed in part but is something like 95 duro so is pretty hard to damage. As you say your swapping the disks (note which is which, high / low), valve and gauge. Use some thread tape or sealant on those even though they are tapered threads and see how you get one.
