Molly - I've also had problems with the ITB at the hip. (I agree most seem to get pain at the knee!)
You say you are not really a runner, but do you do any other sports or lots of walking?
It took me ages to work out what was wrong with my hip...spent 5 months thinking it was a problem with the actual hip joint...and there are several muscles which refer pain into the hip, including muscles in the back.
How long have you had the pain for?
Shame all the previous treatments you have tried have not worked for you and you are having surgery, but I hope it goes well & alleviates the pain.
my ITBS was temporarily cured by an osteopath - got better for 2 months & then the pain returned after I stepped up training for a half and did some hill walking.(i've been told training on hills a lot can aggrevate the ITB & think that might have started mine off.)
Good luck with the op.