I feel your pain! I have had sciatica for the last 3 months, started mildly ramping up to full blown pain, can't sit, can't lie down, can't work/drive etc. When I had an MRI 2 weeks ago, it confirmed a what they call slipped/herniated disc between L4&L5 part of my spine. Apparently this is common for wear and tear in a disc. Having read your post, I have exactly the same symptoms, but I would maybe cautious about things until you have had your MRI results in case it is something different.
As it was slipping out from it's proper place, it has been pushing on my sciatic nerve, causing all the leg and lower pack pain. The good news is that after all the agony, things are getting slowly better for me with the following things:
Physio - specific excercises, ie laying on floor, on elbows pushing up, creating a curve so your back is in a 'u' shape. However, if it really hurts, don't do it, try at a time when the back is more warmed up and not in as much pain. If you have a slipped disc, your NHS physio should be able to show you many more.
Walking / Swimming - gentle excercise seems to help keep pain at bay. walking and keeping moving during the day helps. swimming (not breast stroke) ie, front crawl or with a float helps keep your spine in a position to help ease the disc back into place.
Cold packs - cold compress on the back area at regular intervalls.
Accupunture - not entirely sure if this works, but 48hrs after both sessions I had, I have seen a marked improvement, but this could be coincidence!
Oesteophath - expensive, but if you find a good one who knows what they are dealing with is worth it for their advice.
drugs - I have been on many, but the combinations I find work currently are: neproxin (anti inflams), tramadol (stronger than codine), paracetamol and amytriptiline at night, plus a dose of morphine at night. I am normally very anti taking painkillers but I had no option as it got so horrendus. I am now not generally needing the tablets in the day, but take before bed and during night. compared to a few weeks ago when I was counting the hours to the next dose and pacing the house in the middle of the night in tears for hours.
so, the above may not work for you and you might not even have a slipped disc, but your description is texbook copy of my symptoms. Wish you all the best of luck, it is the most awful condition I have ever had the misfortune to have!