Have finally got to see the physio!
I felt a bit of a fraud as I thought it wasn't necessary, but in fact she confirmed that my boobs are affecting my posture and my spine, and that my muscles are extremely tight across my shoulders and back due to my 5' 3" body trying to hold them up. I also sit in a hunched position, due to the weight of my boobs and possibly because I'm subconsciously trying to make them as inconspicuous as possible, so I slouch with my arms held forward.
So I've been given exercises to do, which so far has made my pain worse, and she gave me a massage which left me feeling really sore! She confirmed that I won't be referred to a surgeon until I've had physio and can prove that it didn't help. If it does help, that's that - stuck with 'em for life.