My previous boat was a mk11 and the polystyrene was held back in place (back against the transom and up against the underside of the aft deck) by a fibreglass retainer. Looking into the bouyancy tank you would have only seen the fibreglass and not the polystyrene, but by putting your hand underneath you could certainly feel it. The polystyrene was actually in quite poor condition so I removed it and replaced it with a bouyancy bag and a number of 5 litre plastic bottles.
On my current boat (mk1) I removed half of the rear tank bouyancy in order to fit an outboard mounting bracket. Again it was not in the best of condition so I replaced it with closed cell foam. I feel more comfortable with bouyancy bags / closed cell foam / plastic bottles that I can easily give a visual check than polystyrene hidden away behind the fibreglass retainer.