Correct me if I'm wrong (and I know you lot will!) but isn't it supposed to be installed groove side down? I walked past a building conversion the other day where they were fitting it tongue side down but didn't like to say anything - first, because I wasn't absolutely sure and second because I could guess the reaction if some old codger passing by called out "Oy! You're doing that wrong!"
Is it a building control issue or just something the new owner will have to shrug over and replace in a few years time?
Nick