Thats very kind Lennart, thanks
I have found since posting that a 1l drinks bottle with the thread cut off fits the aperture perfectly, as do the prebiotic aldi yoghurt drinks. I managed to pump the mat up with one dustbin liner full of air, then the bag popped. A rubble sack was a bit better but a pain to use. I'm now sewing up a tube of parachute nylon and glueing it to the bottle section.
I have found since posting that a 1l drinks bottle with the thread cut off fits the aperture perfectly, as do the prebiotic aldi yoghurt drinks. I managed to pump the mat up with one dustbin liner full of air, then the bag popped. A rubble sack was a bit better but a pain to use. I'm now sewing up a tube of parachute nylon and glueing it to the bottle section.