I'm glad I just read the bit about 5x5 sheets. My pal phoned me to tell me about a company where he said I would get some, and it is in 5x5 sheets.
"Birch Plywood C Grade 1525 x 1525 x 3mm FSC
Good quality plywood generally used for components and furniture framework"
I'd read somewhere that the same ply is used for furniture, so hopefully this is the real deal, or rather, real pine.
I don't really know how I can find out for sure as the guy in the store may not even know, he'll just sell by the catalogue number.
Daveo, the company is called Travis Perkins and as they have over 600 stores in the UK, there should be one near you. There two practically on my doorstep that I didn't know about. No price online, you have to phone your local store.
Thanks all,
Martin.