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The last lot of 100mm insulation I bought was over 20m^2 (7 1/2 full sheet equivalent) and cost me just over £50. I donated some to another buyer as I could only fit 7 sheets in, and on, my car. All in 1200mm square or 600mm x1200mm. I thought that was a reasonably good buy.
There were probably over a hundred ‘equivalent’ full sheets on sale that day. Price depends on demand on the day. Sometimes better than that.
Where from? I want this sort of sale!