My Silentnight mattress is making a noise

Hi, currently trying to find bed & mattress for newly decorated bedroom Purchased a divan last year with SN Microcoil 3 mattress. Currently sleeping on this & although comfortable there's a tiny annoying noise when I breathe in an out as i'm drifting off to sleep, possibly a spring although its not a squeak. Not a major problem but enough to make me want to ensure I get something bettter for the main room. I'm looking for a kingsize mattress & bed frame. Have seen a frame I like with sprung slats (a Birlea bed, "Tokyo") but now worried that the sprung slats might make a noise. Have slept on fixed slats before without a problem. Interested in your Origins 1000 mattress. My questions are: 1) is your Origins 1000 mattress heavy enough to keep the slats down?

(Having posed that question I realise I don't actually understand the point of sprung slats) 2) how long can I expect the mattress to last/continue proving support I am 10st, no partner to consider at the moment. Budget is small. Many thanks Clare .