Ahh, that's definitely worth knowing. Being a sciatica sufferer, that's the last thing I need! Thanks :)
You need to make sure the saddle is fitted properly - flocked level and to the horse's individual conformation - and your leathers haven't stretched unevenly. The majority of (rider) back problems appear well after purchase and are caused by unbalanced saddles, IME, rather than an issue with saddle type or make.