Within reason, there's nothing wrong with using out of date plaster. It's not suddenly going to pear shaped when it reaches that date. If it's been kept out of the damp there's nothing wrong with it. I often buy out of date bonding plaster/multi finish at my local B&Q with no problems, and I get a good discount on it. I have to point out to them it's out of date, but anybody who doesn't notice the date, would pay the full price. It goes off a little quicker, especially the bonding, but that's better than hanging around for ages waiting for it to set. Our local Builder Centre used to give it to regulars instead of dumping it. The quality is just the same. I think Boy Pop, that if Travis Perkins had offered you a better discount, you would have snapped it up, but because they didn't, it's rubbish. Each to their own but I have no problem with using it at all.
Roughcaster.