Well thanks all for your feedback!
We started with a coleman self inflating. Unfortunately it had issues. Mainly it was slow to pack up, and if they get a hole in them (as ours did) then you're in real trouble as they don't compact down anymore. I think it's a fine solution if you're staying somewhere for a few days.
Then last year we made up a foam mattress. It was supremely comfortable. We wrapped all our bedding in it and just rolled it up like a swag. So it was comfy and fast. The only downside is it's huuuge.
So, we're now at the stage where the kids are getting bigger. We used to run the fridge in the back seat. However this isn't our first choice as it makes it much harder to get the second child in (especially now he's bigger). Also the two kids are wedged together which is not always a good thing. Lastly, even though it's tied down as well as it can be I'm just a tad uncomfortable with it being in the cabin with us.
So, if we went with an airbed I reckon the saved space will allow us to move the fridge behind the cargo barrier into the back, and for the kids to have more room.
I'm tempted to give an airbed a go and see how we like it. This next trip is northward bound into warmer climes so that should be a positive for it. I might just ruminate on it for a bit but there isn't much time left...
If we go ahead I'll be sure to post back on our experiences.

