A good physiotherapist will treat the sciatica and give you exercises. Take up pilates asap and yoga but with well qualified and experienced instructors.
I'm not surprised the medications aren't dealing with the pain, my experience is they rarely do but are guaranteed to make you feel dreadful. Codeine is addictive, paracetamol has recently been found to have no impact on back pain,amitriptiline seems to have unpleasant side effects from the experience of people I know who have taken it. Naproxen is an anti inflammatory and 500mg 4 times a day should be helping, if it isn't, stop taking it. Did your GP advise you to take the anti inflammatory after food?
I am not opposed to medication but you seem to have been given a lot for sciatica.
Boots sell Tense machines which can help with sciatica and other back pain. Hope you get some relief.