You can bet they will last. They are for hot or cold water supplies, so definitely going to last on cold to cisterns. They are not made of ordinary rubber.
They just require a little nip up. You might find they are a fairly tight in a copper tap fitting and need carefully pushed into the shoulder with a small screwdriver.
You should be able to get them, or order them from any merchants that stock Hep. I think they are supplied in packets of 10 or 20 with some Hep fittings.
Maybe a good idea to speak to a Hep rep if merchants don’t oblige
