@ChrisEyles well I popped to my local Tesco (online showed stock) - some were priced at £12, some at £6 so I took a £6 and it scanned as that - though all would I suppose. As milkfloat suggested, as presented rolled up it does appear to be near the size of a traditional foam mat and it comes in a long bag - with two straps to hold it rolled inside.
It will though pack smaller using the traditional Thermarest system I outlined above. How do I know - well I got down on my hands and knees and rolled it on the store floor. Won't maybe pack as small as my Thermarest but then while the same size top to bottom side to side it is a little bit thicker. And, lo, I would say it is more comfy than the Thermarest I paid £40 to £50 for a few years ago. It doesn't have the Thermarest's quasi semi fabric finish but is smooth and plasticky but I don't think I'll be rolling off it, especially when it is used inside my bivi - nowhere to roll
Do make sure you keep your receipt Chris in case of problems - I checked mine out at an auto till and forgot to take the receipt.
All in all, especially as your bivi trips don't seem to be long haul at the moment I'd say it's ideal for your purposes as an intro to mats. You can always get something posher later.
edit ps - I paged you upthread of your comment to say they had gone to £6 again. You not sleeping well
