Agapanthus, have you thought about free standing shelving, i.e. putting legs on the ones you have made? My train of thought brought into question the inherent strength, or lack of, of the greenhouse walls when you've filled all those shelves. If this doesn't appeal, I had success last year with using very cheap 1" x 1" steel brackets (about 6p each) to anchor a temporary shelf, then suspending the front of same with hanging basket chains, a bit like a drawbridge, to a point further up the wall. I could send you some diagrams but I haven't fathomed how to do stuff like that on here yet. Hope you find a way.
Geoff.