I was in B&Q yesterday getting some silver bubblewrap for wingcovers for a new glider and sat next to it were these big blocks of foam that looked just like 6mm depron !
I picked up a pack just in case - contains 8off 800x600mm sheets. I only opened the pack 5mins ago to check and it definitely looks and feels like depron (I should know, I cut tons of the stuff on my 'amazing depron machine'
).I cross-checked the price against what I pay from the distributor - B&Q is £3.35/m^2, distributor is £5.35/m^2.
The sheets are smaller than the normal size of 1000x700 for 6mm white, but most of the parts are < 800mm and if not, you can easily join it with a sawtooth joint and some uhu-porn or xiao xiui (really
) 
No doubt, they'll stop selling it now - or it'll all be bought up and sold on fleabay at twice the price
BTW, it's Diall branded and made in Germany.
Cheers
Neil
)