What should I replace a Relyon Pocket Sprung mattress with?

Hi
We have a Relyon 'Country Pockets' pocket sprung mattress 27.5 years old with a sprung divan base. The mattress has 'spread' (ie got longer and thinner) over the years but is still very comfortable but my husband wants a new one! He is about 15.5 stone and I'm about 10.5 stone. We don't want a really firm mattress but medium or maybe medium soft (for him!). We're not keen to spend more than £500 but would if really necessary. I find the range of mattress types available on the internet very confusing but I am inclined to stick with a pocket sprung - I just don't know what manufacturers to trust (if any!) and how many springs is enough! Do you think one of your Origins mattresses would be suitable? I don't know what our current mattress is made of but I doubt that it's natural fibres and since we've been happy with it, I'm not inclined to change. I'd be grateful for your advice.
Carolyn