Welcome, Suffolknovice!
The basic rule of thumb on hedges is that you want to plant them 1/2 as far apart as the planned mature height. That gives the fastest fill-in, without some dying from competition, later. If you want them to grow a little over 7 ft. tall, you have the right number. If you want a shorter hedge, either get more, or expect a longer fill-in time than you would have with closer spacing.

Thank you so much! I was planning a lower hedge, maybe 5ft so maybe I should pop back to the garden centre & see if they have any more on offer (they were 5 for £20 & didn't have enough for us to get 15 at the time!)
It's so good to go on a forum that works!!!!

Good luck, then, Suffolknovice!