So update time.
Phoned them, got moved to the current 1Gb/1Gb deal for £26. Was paying £35 for 500/500. That was made up of £30 for the 500/500 as 1Gb wasn't on offer when we originally got it + price rises over two years + £4 out of contract. Not exactly the same as the website package as no new router, they said they only send if something is wrong with current. Current one is fine, so I don't care.
CGNAT, took a while because of waiting and the tech person had to keep putting me on hold to go to someone above them, but short version they said the account is flagged to not use CGNAT and I can keep it like that, I don't have to move to cgnat with the upgrade.
Apparently it will take 3-5 days to make the change. I guess it will just go down then pop back up at 1Gb.
Other than that, I'm not the account holder, but CF was fine to let me speak once the account holder had given all the details and said about passing over to me. I know some companies can be funny about that, so just thought I'd add it.