Well... I sucked it up and called EE on Saturday, I'm not joking when I say I hate calling people, but you know, £1300+ worth of phone and yey, I've made about the only call I will make all year! 🤣 (it's certainly true of last year!)
Anyway, it was a breeze, only three options on calling, can't remember what they were, none seemed to fit the bill, so I selected '2' which was for tablets and broadband (?) I figured it was fine, it was about data and not the phone... 🤷♂️ I told the guy exactly my set-up (which was Pro Max, Series 4 with plan and Series 5 with no plan - both watches paired with the phone) - I slipped in that it was a real pain last year 🤭 - He told me I was really lucky, cause he knows what to do, he's a tier 2 and fixes all the crap (his words) when the tier 1s get it wrong!. - I'm only guessing, but maybe option 2 got me exactly where I needed to be! - Anyway, he told me to unpair the Series 4, he literally said not to worry about the keep plan business so I did nothing, and while it was unpairing, I read him the SEID for the series 5, he fiddled on the computer then told to add the unused EE plan on the series 5 (via the watch app on the phone) and done! He said it should show up within 24 hours, but to be fair it was there virtually immediately. He then 'chatted' about old phones for about 10 minutes after everything was done, pleasant enough but I wasn't entirely sure when the call would ever end - Haha. #Done.
Battery...
For me Apple Watch performance has been fine, near identical useage throughout series 2,3,4 and (so far) 5 - each new watch has seemed to give me less 'day' to work with, I think my best was series 3 and with a day being from about 9am to 11pm I would end with 75% battery. Series 5 has been a bit worrying up until Saturday - If I started the day fully charged at 9am by 11pm I had about 20% left. I had messed around turning certain features off (raise to wake and background app refresh), but that's not the point of the purchase. I was a bit wary of applying the Data Plan in case I had to return it. BUT... I've just had a massive improvement. I don't know what caused it as it collides with ios 13.1.1, and applying the data plan. I was 100% charged Saturday at midnight, went to bed wearing the watch (theatre mode) woke at about 9am turned theatre mode off immediately and pottered all day in the house (arms exposed/no sleeves), raise to wake, plenty of notifications, infograph modular... and upon needing to charge for the night at 11pm I had 33% left after 22 hours away from the charger. I may see what it's like with Background App refresh on, but I'm not sure you need it, the apps on your screen are auto refreshed and can't be turned off if you are using them on the active face.
I'm happy

I can probably slip back in to my charging schedule of 11pm to 12pm and 8am to 9am before work.
Sorry, gone on a bit, but it might interest someone out there... hopefully