Depends on current package really, but generally its £7.50 per month and an activation charge of about £49.95.
I must advise that even though theres a cable present in the conservatory, donesn't mean its connected (it may have been disconnected by an technician when installing current services) however if your positive that connection remains Virgin might supply a quickstart for the additional box.
To do this call them up on 0345 454 1111, be prepared for a long wait and potentially told no to multiroom as they are currently holding all multiroom orders for new or upgrading customers.
If I've helped let me know 🙂
10 year's managing 9 Virgin Media accounts, and things never got any better, closed last 4 accounts since January 2021 :L