Superkingsize zip and link mattress

Hi,
What should I be looking for regarding a super kingsize zip and link mattress? I've been recommended your site by my college for your helpfulness.
Some background:
I'm 5ft 6, my partner 6ft 1
I'm 9stone, my partner 13 stone.
We don't have a specific side of the bed and swap and change as we feel.
We generally sleep touching.
At the moment, we have 2 little people in our bed every night (a 1 and a 5 year old)!
The only reason for choosing a zip and link to be able to get it up the stairs!
I've bought a superking size divan from M&S - the only one I can find that isn't on a solid base.
At the moment we have a Kingsize Feather & Black pocket sprung mattress - not firm, not soft - we like it.
Our budget is £1,000 or under.
My concern is that we'll feel the zip, and I'm hoping we can cover that with a mattress topper?
Please let me know if you can help!
Thanks
Jane