Need mattress to help with insomnia - Help!

High guys. Well firstly I wanted to say thanks for helping me learn about mattress's. I have not had a decent nights sleep in over a year, normally 4 hours max, awaking every hour thereafter. I realised that the only variable left to test, was my mattress and bed. Both were bought at the same time, my insomnia started.
The main problem I have with my current mattress, is that I feel like I'm drowning in the damn thing. Its got 1,400 springs and a memory foam top (rated as medium firm) but I would not class it as this. Its soggy as hell. On the plus side, I do not get lower back pain from it (I can suffer from this very easily) but I can't help but feel slightly depressed every time I come to lye on it and wonder if this is effecting my sleep. My bed frame is also some cheap naff thing, with slats so weak I wouldn't rest my dinner on them. I'm 6'1, 78 kilos (and probably rising as I work out a lot, but I will probably never exceed 13stone) my budget is £300-350 and I'm looking for a king size.
So today I finally snapped and said enough. Went to F&B's to try some mattress's. The first one I lay on was like heaven. I just wanted to pass out there and then. It was £275 pocket sprung but was a little too firm (I'm guessing it was 1000 sprung). I instantly noticed lower back pain (I have hyper extension in my lower back). I then went to another dedicated bed store and found a really nice mattress, that was really supportive/firm but had a nice soft top (it was button topped only). It was Sleepeeze Ortho Supreme 1000. It was up at £450 but after haggling, the guy said I could have it for £300 (which ironically sent alarm bells ringing). It has no natural stuffing (cotton etc).
I then went to another store and they had a similar one for £450 (different brand) but wouldn't budge on price. Said it had natural fibres, which probably meant they had 1% in and a %30 mark up for it lol. So this Sleepeeze one was definitely the best but I told the guy I would sleep on it (sorry bad pun).
On getting home, I tried to look up the product, but had little success. My concern with it, is that the nice cushioning on the top, won't last long (probably cheap man made fabric, losing its bounce in a couple of months) and I will be left with a too firm mattress and thus lower back pain. I have also noticed the £300 deal is not as good as it seemed, after going to Mr mattress and seeing better spec'd Sleepeeze mattress's at the same price. These were the other two current options form that website. The 'Sleepeeze Concept 1400' I can get a king size for £299 (it claims to have wool and silk filling), which compared to the 'Sleepeeze ortho supreme 1000' seems to be a better deal (although how much wool and silk is anybodies guess). There is also the, 'Sleepeeze backcare luxury 1400' (kingsize) coming in at £331. Again claiming silk and wool. The only issue is, the Concept 1400 is its a 'medium' and the Backcare Luxury is 'extra firm'. I think the extra firm will be too hard (unless I add a topper) and the 'concept' might be too soft.
I looked at your products but I don't think I can swing Over £350 (unless you have any returns you price down accordingly) as I would prefer yours by all accounts. I don't have a big budget for this and with over a years worth of poor sleep, I'm getting desperate to find the best solution for me. I want something soft enough to avoid lower back pain, but firm enough I don't feel like I'm drowning. Do I go for the extra firm and add a topper?
Or the medium and save the extra £100 I don't really have for a topper?
Plus its £40 cheaper to start with. Will either last that long?
and do you know a better quality product around this price range?
I'm also concerned about slatted beds with pocket springs. What are your thoughts on that too?
Would really appreciate the help guys, money is tight and I really need to get the best out if this. Again I would like to try your products, but I think they are just out of my price range. Help!
lol Best Regards Rich >