Ok so I did an online chat with one of their reps & they said 99% no charge likely even if there is no line/connection entering the flat. & if they will need to install a fresh line or similar - great if that turns out to be true!
I was very impressed with the knowledge of the chat rep (Toby), I have joined myself on the 67mbps package (£20 social tariff).
They will also 'pay' the 2 months I have remaining on my previous ISP, by including this as credit on my account.
Previous ISP of over 9 years couldn't match BT's offer - quoted £24 to continue with them on my standard 67mbps non-social package (they had a UC-only criteria for a slower £20 social tariff, which I wasn't eligible for), so I left. Hope it's a good decision, as I had very few issues with them & their support is UK-based.
Now unto getting my friend sorted in the next few days!
Thanks again for all your help guys 😀, much appreciated 👍👌🤛
P.S. I shall have to mark this post as the accepted solution, all of you contributed to me getting there somewhat with the final answer, which to be fair wasn't straightforward & needed inside/direct knowledge. Hope that's fair
