Thanks to those who made suggestions on where to source a new mattress. On measuring our old one I found it is 10cm thick. The thinnest of mattresses online seems to be 20cm and they get thicker. I will ask the company about weight of a new one, but other than potentially being heavier and more comfortable, is there any downside to a thicker mattress? Our storage is under the bed so it needs to be able to lift - would this be problematic? We can't try a new one out so will just have to make a decision. But it made me curious about how thick they generally are and can I increase the depth without causing problems?