eBay is where i got mine from, tho it hasnt arrived yet, I'm hoping it'll be here tomorrow. Search eBay.co.uk for polystyrene sheets. There are lots of sheets 25mm thick, but i wanted mine to be thinner, so i found some that were 10mm thick. 25 sheets of 600mmx400mmx10mm cost me 12 quid including postage.
I am going to fix the pieces together with cocktail sticks. That should keep it nice and steady for when i put the grout on it. I dont like the idea of using any glue really. I would only use aquarium sealant/dow corning but this would be a bit too messy for this job.
I need answers to my questions!!!!!!!!