I knew the tiles you were on about Xanadu - they're almost like plasterboard in consistency - in fact, if I look up now I get an eye-full of them - but as ever - I was impatient to get on with things
The polystyrene board is improving with age however - as the PVA glue is drying out through all the layers, the whole thing is becoming firmer, and the pins are sticking better into it. I'm more and more pleased with the results.
As an additional point however, I had pinned my plans to the board, then covered the plans with a thin sheet of polythene to stop the parts sticking to it... Unfortunately, I had only waited 24 hours before doing so - and I've found 1 week on that the solvents for the glue have seeped upwards and then been caught by the polythene, leaving my plans damp, and stained... And somewhat pungeant!
Drying it off with a hairdryer seems to have solved most of the problems this morning tho - I shall take the hairdryer to the board before re-attaching the plans this weekend