William Night Mattresses on slatted bases

Hi,
I am replacing my children's beds. After researching on your site (which is fantastic by the way) taking into consideration my budget £600 so I know I have to shop around, I thought I would like to just buy mattresses of a better quality as both bed bases one single and one double are still in good condition (both are slatted).
I have found William Night mattresses 2000 pocket springs with a latex topper at £700 for one single and one double. I would be interested in your views on the manufactures quality and do you think they would be suitable for slatted bases. My eldest child is 6ft tall and approx. 12st (double), the younger one 5ft 2" and approx. 8st.
Any advice would be gratefully received
Mandy