Ok so I've had another go at trying to make some progress today with a few interesting findings:
- On my iPad I had left it that two cards were there setup working ok, I didn't know how but when I added Card 3 it always screwed up all three cards' addresses. Today I added Card 3 and it let me remove the old address!... BUT... restarting the iPad brings it back but always lets me delete it. I've tried restarting and force restarting and both cause the old address to come back. When I then go to change to the address it's still odd: tapping on new to move the tick and coming out goes back to old address but just swiping to delete old address remove and not having any ticked but going back removes old address on all cards instantly and ticks correct new address.
- On my iPhone no progress at all. Fully removed all cards and tried again but no combination fixes things.
- Lastly, I tried something I hadn't before: my work iPhone has never had Apply Pay on it nor is it in any way linked to my personal iCloud account (it has a work only iCloud account I created to sync a few bits). I added troublesome Card 3, validated by text message and it instantly worked - it obviously had no address info so I filled in and it's staying through restarts etc. - no issues
This essentially means for me it's either a device issue (may see progress when new iPhone but I'm not keen on hours explaining all this at the Genuis Bar, especially as it didn't work for the person further up) or, most likely, there is something in my iCloud account screwing this up (maybe linked to device also, explaining why I've almost fixed my iPad part).
Hopefully this helps others get further along to testing stuff!