So I bought one metre of PVC fabric from John Lewis and set to to make my own.
Firstly the PVC I bought is not quite as thick and strong as the original bag, but I didn't realise that until I got home, and it will be OK if I take care with them.
I measured the original bag, and with the help of a biro on the wrong side marked the fabric (why do they never cut straight and square when cutting from a roll so you have to square it up before you can start?). I managed two bags plus some new handles for the old one.
My new shopping bags and iPod case (old bag at the back)
Then I hit the problem of sewing it with my machine - it is so sticky it will not move through. A quick internet search found the answer - sandwich on each side between greaseproof paper which is pulled away after. Job done!
It was a bit of a fiddle and I ended up with torn paper all over the floor but I got there in the end and I now have two new bags plus a repaired one. There are some smaller pieces of PVC left and I made a little zipped bag to hold my iPod and headphones. I will probably make some more similar bags with the rest when I get round to it and can think of a use for them!