Sorry for not getting back to you all sooner - I've been bouncing around the country doing professional firework displays.

@martin54 - It's definitely rigid PVC and not Foamex. According to the web Foamex has a density of between 0.5 and 0.7 (and so will float). I've just measured the density of some offcuts of my material and it comes in at 1.4 which fits nicely with it being Rigid PVC (1.3 - 1.45).

@martin54 - the holey mat was new for this sets of cuts.

@Jonathan - thanks for the suggestion although I'd hope that the holey mat would have the same effect although I can see that a seal just around the edge might be better.

@martin54 - thanks for the link to the vacuum pumps. Although quite expensive it's a price I can stand if it'll do the job.