Thanks to the people that commented on my question (problems moving a Cellular plan from series 3to series 4), I was really hoping I wouldn't have to call 150 (EE) as I hate call centres LOL, but I did, I had to, and after 45 minutes on the phone to a lovely girl who took me through several steps, I shall have to wait 24 hours to see if the steps we took allow me to pair my watch to my existing watch contract. Wasn't too traumatic if anyone else is put off by having to call them, I had my looooong EID number ready, she was mostly running off to a tech person and popping me on hold, and a significant amount of time was spent unpairing and re-pairing the watch.
(I was her first call with this problem, she took yesterday off deliberately LOL)
...Fingers crossed all goes well in 24 hours!