Yep, best you can afford.
We've had a few inc an ikea memory foam one which i hated it - after 2yrs it was sagging and by the end of the 3rd year i was in so much pain it went and we bought a really expensive one (well, i thought it was really expensive at £1,200 and omg it was/is fantastic (It's on Scarlett's bed now - lucky girl!))
Where to buy from tho, i have no idea as we ended up getting ours from Bensons which is just like any other mattress retailer i guess BUT... when the time came to get one for our super king bed i knew i wanted the same mattress and actually bought it from an ebay for £400 which is a bargain, especially for a super king.
I would suggest you do some googling about spring density etc and that will help you eliminate SO many mattresses. I cant remember now but theres something about 'x' springs per inch or whatever which helps determine the support quality. A mattress with less springs puts more pressure on the springs so they sag/break etc where a mattress with lots of springs will distribute the weight better.
No one can really suggest a mattress to you as everyone likes a different kind. My hubby prefers a firm one, i like medium so we have a medium-firm fully sprung memory foam mattress with quilted cover.
Yeah so, once you know the spring do'dahs you can have a good bounce about in store then search for other mattresses online etc and not feel like you *have* to buy one in a shop.
Good luck, i really hate mattress shopping as it can be very expensive and you dont know if you like it until you've bought it
