A bit of a thread resurrection here!
Whilst answering another coolant query in the technical section (1.8 this time) I stumbled across this:
https://rimmerbros.com/Item--i-CDU3858OS
I don't know if it's been spotted or mentioned before, or if anybody has tried them, but it's another potential solution to the persistent o-ring failures seen on the KV6.
It depends just how oversized it is I suppose, but I feel this could be a solution for my problems and thought I'd share it just in case it helps anybody else.