Thanks for all the advice, I've spent some time looking at the various options, and in case it is useful for anyone, this is a summary:
BabyBjörn Travel Cot Light is the most expensive option, but is very well reviewed. Unfortunately, it is too big to put in most suitcases and is still 6kg, so perhaps this would be a good option for people who have cars, but are still very much space constrained.
Koodi pop-up and the like. These are lightweight although don't necessarily pack that small. They are not suitable for babies less than 6 months though, so out on that count for me.
Phil and Ted's sounds like the perfect option. It weighs very little, packs small and includes a thermarest type mattress. Unfortunately, it is quite poorly reviewed with people complaining that it is hard and time consuming to put up and that it falls apart. The latter point is something of a concern as it was mentioned a lot. Also, they seem to have updated the design recently which is not proving popular.
LittleLife Arc 2 - this weighs very little, looks quite sturdy and can be pegged / tied down. It doesn't seem to be as popular as the other ones, so there aren't as many reviews - it seems to be more highly regarded than Phil and Ted's, but less than BabyBjörn. My concern was that it was still quite balky and came in a backpack.
I'm tempted to go with the LittleLife Arc2 given the comments by Janek about splitting it all out, but it is still £90, so a bit of a gamble for a mini-tent.
Janek - if you read this thread I would be very grateful if you could explain how you used a thermarest, because I'm assuming it is tricky to get one the exact size. Am I right in thinking it came with a bulky foam mattress which you ditched for a small imperfectly fitting thermarest?