In reply to andrew ogilvie:
Exped faulty mats are a widely known issue (note the recent UKC gear review of mats there wasn't any consideration of robustness / durability), forcing Exped to have to change the baffle design. The older design had a much thinner weld, which has now been beefed up.
It would be interesting to know if your mat is the old or new style?
My Exped UL failed this way twice, I sent them back to direct to Exped UK's agent each time, once out of warranty, and received a new replacement mats. Both times they failed when I had inflated them firmly and just rolled over heavily on them (and I'm not a lardy!). I had no qualms about sending back out of warranty, they cost an-arm-and-a-leg and clearly weren't/aren't fit for purpose. Try it ,after all you only have the postage to lose, and do let us know how you get on.
On the last occasion I lost confidence, flogged the mat new and unused, and brought a more robust competitor offering.
Having said that my partners down UL has lasted many years (albeit it with an intermittent leaking valve), but she is a 8 stone whippet.
I'm sure Mountaineering Designs in the Lakes would re propose the down.
Post edited at 10:20