I use an exped synmat like already described. Dougie uses a thermarest neoair which is almost the same thing, but the thermarest has a stickier surface to stop you slipping off the mat if you have to pitch on a slight slope. The thermarest is also very noisy I find compared to the synmat. The synmat is made out of a material that is very easily stained however.. Just a wee bit of condensation or a wee tea spillage and it is stained forever
We also usually use foam mats from October to May - partly for insulation, but mostly to help prevent condensation on the inside of the groundsheet - works rather well
At the beginning of April, I would definately want to take both..
I have a spare self inflating mat that I never use, just gathering dust and taking up valuable space in my tiny flat. I think it is just a 3/4 length one if I remember rightly, but used with a foam mat, would be rather good. If you want me to post it to you, just PM me your address