Hi Wogga,
The comfort mattress is about 40mm, the top mattress measures 40mm when it is still in its bag (this will probably expand a little when the bag is off but no more than 50mm)
We tested sleeping overnight in the bottom bed and I found I needed (the wife needed) a topper for the bottom comfort mattress as well. I bought a double Panda Bamboo topper 50mm from Costco about £80 before the Duvalays arrived. I also bought two single smaller standard Duvalays to use side by side on the bottom bed and this works very well. This all takes up a bit of storage room though but I think it takes priority.
@Grumpy Grandad.. I bought exactly the same headrest mount to use on an iPAD with a small netgear 4G router permanently installed and running in the Cali (that way it supports the Nest 2 Carbon Monoxide and smoke detector as well).
Cheers