Memory foam Mattress Topper - or new Mattress?

We found standar sheets fitted our old double with a topper ok but need deeper sheets for our new mattress with topper built in

we have a sprung mattress with a memory foam layer included on top. I also put a thin fabric topper on as it is washable. It is lovely and comfy although not sure i would go for one again as it does get warm and i hate being too warm in bed. Our fitted sheets fit fine.
When we get round to doing our bedroom i will definately be getting a kingsize - need space for the cat!