The GP was very helpful, reckons he should be on paracetamol 4 times a day (he's not been taking it regularly, only for headaches etc), and to top that up with the oramorph when needed. The 2.5 to 5 mls can be every 2 to 4 hours, and if/when he's taking it regularly to 20mg a day they can switch him to the slow-release version.
One thing we don't really understand is how the leg twitching triggers the pain, which is how it seems to him - we understand the twitching is from nerve damage caused by the spinal compression....I guess maybe in truth the same thing, "something" pressing on his sciatic nerve, causes both.