We never worry about sell-by dates on hard cheeses. If they look good and smell good we just scoff 'em, often three months out of date. Soft cheeses are very different. I've had mouldy mozzarella just two or three days out of date, and camembert and brie die quickly. Blue cheeses are simply not meant to be kept for any length of time. I haven't tried that double Gloucester but I'll look out for it.