I have the same matt, I know you can do a DIY repair, but personaly I would send it of to Cascade in Ireland, if your lucky they will fix it under the life time warranty, if not it could still be worth paying for a pukka repair, I would imagine a new matt is around £80 these days.
I've just repaired a two inch cut on my sim using McNett tenacious tape, I used the clear tape and you just cut a patch about one inch bigger all the way around the cut, you round the edges and stick on. I left it overnight before inflating and its perfect and apparently the patch gets stronger as time goes bye.....
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