Persistant back pain.

In approximately spring 2007 I started to get mild pain in my lower back just to the left of my spine near SI joint area. I am not sure what caused this, it could have been putting on my socks or jumping off a gate. I also used to feel an uncomfortable sensation in my hamstring / rear of thigh area, not pain but uncomfortable, this lasted approx 4 months then faded and went away.
Then in Nov 2008 I again developed the pain in my lower back. I was outside shifting wood which meant bending and lifting not very heavy, I was also pushing a heavy wheel barrow. In the evening it felt painful again in the same area as before.
As time has pasted the pain now radiates from lower back as above into left buttock and down left leg, rear to left of thigh, and left of upper calf.
My lower back feels solid and stiff in the mornings and pain down my leg, this does tend to fade slightly but is constantly stiff.
The pain is helped by lying on my back flat.
I used to have pain in lower back when I was sitting and lowered my head, although this is no longer a problem.
Sitting down, eg in a car is ok but after a while starts to ache.
The most pain is caused when I stand up. Normally can be about five minutes and it starts to be very painful, this is relieved by sitting.
I saw an Osteopath who suggested Spondylolithesis (spelling??), I arranged an xray which proved negative for that, the radiographer did mention that my spine had lost curvature?? But I am not sure what may cause this.
I have seen numerous Physio's and Osteopaths who have provided stretches of hamstrings and core stability exercises. But the pain although not as severe is still persistant.
I took paracetamol / Codeine for nearly a year, I have stopped this as they were making me feel unwell. I am now taking low strength Iberprofen / Codeine, this takes the edge of the discomfort.
I frequently have disrupted sleep and get woken by pain.
I have never had back problems before in my life.

I am getting married this October (2010) and dont want the day spoilt by this back pain.
If anyone could offer some advise it would be greatly received.
If you need more info please ask.

Thanks in advance
Simon.