Memory Foam Solution

Even I'm getting bored with this one - but here's a possible solution, which was inspired by a kind person who commented on a previous post - thanks Tana! - and if you can't afford to replace your double bed with twin beds and different mattresses, it certainly helps.
This is what you do.
Buy a common or garden Yoga Mat. In the UK, these are available for about £10.00 in Tesco's but you can get them anywhere. Go for the smooth ones, not the lumpy ones. Not too thin either.
You're looking for a good layer of foam, but not memory foam! Stretch out the mat on top of the dreaded memory foam mattress. Then cover the whole mattress with a blanket of some sort, and then on top of that, put a good thick mattress topper - but not plastic in any form, and certainly NOT memory foam! - you're looking for cotton or fleece or something similar. Again these are available in most bedding shops, some supermarkets and online. The bed may be looking like something from the Princess and the Pea by this time, but this worked for me. Cover with a cotton or linen sheet, and make the bed in the usual way. The trick is to put as much as possible between yourself and the dreaded memory foam. Not only does this help to ease the 'quicksand effect ' - you don't sink into ordinary foam the way you sink into memory foam and yoga mats are designed to be quite firm - but it also helps with the heat, since the blanket and mattress cover don't 'draw' in the same way as the memory foam. It isn't the perfect solution - but it certainly helps and I'm sleeping properly for the first time in a year and a half!