Mattress type and firmness

Hi,
I really need some advice on a mattress, please.

I bought my current mattress from a bed shop in September last year and it is an 800 pocket sprung single mattress with a firmness rating of 4 out of 5 (5 being firmest) I weigh just over 8 stone and sleep on both my back and side. My bed base is a wooden frame with sprung slats.

I have been suffering badly with a bad back for the last year or so, mainly effecting lower back and it can often affect my hips and pelvis. Laying down and sitting down has made the pain a lot worse over the last 6 months. Because I'd mentioned my bad back to the sales guy he recommended a firm mattress for me. What he failed to tell me was that the single mattress I purchased would end up feeling a lot firmer than the double I tested in the shop.

My back pain is still being investigated for a cause but after reading a lot of info on your website I am starting to wonder if my current mattress is playing a big part in my pain. I have also been using a polyester mattress topper for a few months which sometimes helps but not a great deal.

What type and firmness of mattress would you advise is best for me? Please, can you also let me know if having a slatted bed base makes a difference?