Before spending money on a mattress topper, see whether using a spare duvet under your bottom sheet makes enough of a difference. I was considering buying a topper, but when I was sorting out some duvets that were not used any more, I had a lightbulb moment. I usually put a blanket between mattress and undersheet, but the next time I changed the sheets I used a single duvet instead - it was big enough to go edge to edge on my 4ft 6ins bed. Result - very cosy and comfortable, and it evens out some slightly lumpy bits. Best of all, it cost nothing.