Ouch, do you know what's causing the pain - is it that the muscle is in spasm, or do you have pinched nerves underneath?
Have you ever been referred to a pain clinic, and tried things like trigger point/nerve blocking injections? There are plenty of nonsurgical options for pain relief that can be tried, but you need to see a specialist pain doctor rather than a physio for many of them. Do you take any painkillers? If your pain is neurological in origin, many of us have amazing relief from a drug called gabapentin. Similarly, amitriptyline can work wonders for muscle spasms.
It sounds like your scoliosis has been stable for years (otherwise it would have progressed a lot by now) so I doubt any surgeon would want to actually fuse your spine, but if you have a trapped nerve or bulging disc then a tiny surgery could help enormously.
If you're still in that corner of England, it might be worth getting a referral to the Pain Clinic at Oswestry. This is headed by a Consultant Anaesthetist called Dr Gaspar and he is brilliant. He can also get you onto their Functional Restoration Programme, which is 3 weeks of intensive physio, hydrotherapy and all sorts of methods which can really help.
Exercise is usually great for pain, but after all this time I think you should maybe see a doctor again, as you might be causing more damage rather than helping things