I have a similar issue in that I have a double sofa bed in constant use at the moment. We purchased a really good quality sofa bed that was designed for everyday use but still the mattress is quite thin - I guess it has to be because it needs to fold up. We got a memory foam mattress topper like you have there, it is like a thin mattress and can roll up when not in use. It makes the world of difference. I think we got ours from Emma but looking on their website i can't find it now. I will keep looking but from memory it was about £150 and it is bright yellow and white (if that helps you find the brand). Totally worth it for long term use. I don't think a "normal" topper would cut it really.