I've been there too. Cutting down very slowly is definitely the easiest way to do it IMO - but you do have to be very disciplined about it as it's very easy to give in 'just this once' if you have a bad day, headache, etc.
If you aren't exceeding the recommended dosage each day, I would think it's unlikely that you have damaged your body/liver. Many people take paracetamol for many years with no problems as long as they don't take more than 2, 4x daily. The codeine also doesn't generally create huge long term damage other than constipation, and of course the withdrawal symtoms if you stop. Is there anything else in solphadeine?
The poster whose DH ended up with a perforated ulcer was, I suspect, taking codeine with ibuprofen (not paracetamol) such as Nurofen Plus? Ibuprofen will knacker your stomach big time if taken long term as happened to him.
I was taking a lot more than you are, and stopped in the end with the help of the local drugs team, referred by my GP. They helped me wean me off very very slowly which was brilliant for me. I was urine tested regularly so couldn't cheat, which is what I needed.
I would have no chance of ever getting opiated from my GP now, as has been said already. But when I had my DS2 I had a c-section, and they almost forced morphine on me despite me explaining exactly why I didn't want it! It was bizarre. It happened again when I was admitted this year with something - a Dr said, 'I'm just going to give you some pain relief' and almost had the syringe in my bloody arm before I could say I hope that isn't morphine cos I don't want it! Again I clearly told them my medical history on admission.