Which mattress do I need to buy for £1000?

I am about to order a Hypnos shallow sprung divan base and wish to buy a suitable mattress. I am about 9st 9 and my husband is about 11st 8 on a good day. I wish to buy a mattress with as close to 100% natural fibres as possible for a budget of around £1,000. Our current mattress is. Silent night Mirocoil 7 with a built in topper. We have not been happy with this mattress as it has developed a hump down the middle and I have on occasion woken with a lower back ache.