DerbyBorn wrote:
I need 2 O Rings that seal between the shower cartridge and the brass body.
A kit will cost about £18 and contain things for later models that I can't
even make use of. Hoping to buy a couple of suitable sized o rings - what
material should I be getting to cope with water and the temperatures?

The material does not matter at the temperatures found on a shower. They
only become significant with superheated steam when viton should be used.