This is it, it's big enough for 2 torts to bury in. I paid £37 but see they are £20 now and £44 in Homebase
http://www.diy.com/departments/elsa-...1351161_BQ.prd
it looks like a complete tub but it's made up of individual panels that just click together, You can leave one panel out for an entrance. They are very sturdy.
Celotex sheets were quite expensive at £36 for a 8ft x 4ft x 3 inches thick that's 2400 x 1200 x 75mm or thereabouts. They come in 2 inch (50mm) thickness too. B&Q have smaller bits of about 4ft x2ft x 2 inches for about £7. I'm going to make a better thing for next year.