You've got a few options available. First decision you need to make is whether you want a closed cell foam mat or self inflating mattress. The dutch army mats have a good reputation but are quite narrow compared to british army equivalent. I like using the CCF mat all year round as it is light, reliable and robust and keeps you warm in all British seasons (in my opinion anyway!). The downside of course is comfort, but if you fancy yourself as a bit of a bushcrafter then you can improvise when camping in the woods by making a leaf litter mattress or raised bed. I used a Thermarest Ridgerest CCF mat and have no qualms in using it. The ridges also help to keep you from sliding about.
In terms of self inflating mat, I'd go with the best brand you can afford. I'm liking Multimat, they're a pretty good reputable British brand who are also known to supply the British military. I have a Multimat Summit Expedition 38 which is 3/4 length and very comfy indeed, it only cost me £30 online but I've used it quite a few times now and have always slept comfortably (and warm). I bought this mat to replace something similar that I got from Go-Outdoors, you could easily see the difference in quality as the foam and fabric used by multimat seems more robust. If you fancy a bit of luxury then combine it with the CCF mat and you'll be even better for it! (always a nice option in the winter) The other thing I like about the multimat is that the stuff sack has a fleece lining which can be turnt inside out and used as a pillow.
The key with the self inflating mats is to not over-inflate them, that's the mistake that I believe many people make and then moan about them getting a puncture so easily. They don't need to be as rigid as a surfboard to give you a comfortable night's kip.
