I've been looking out for a good price on thermarest matresses for some time - specially the guidelite as it's a compromise between light weight and reasonable thickness.
http://www.mickiesplace.com/index.htm has a number of thermarests on sale at the moment. I've just ordered a full size guidelite for US $ 78 inc shipping to UK. That's about £45. These are normally £70 odd in the UK.
Insurance is available as an extra.
The equivalent item is also available in the UK on e-bay right now but at £55
These are the best deals I've seen in several weeks. :-D Cheers.