Being from england you probably get resin / compressed weed.

I lived there for 2 years and finding good quality weed was really hard - i eventually had a mate staying in amsterdam post me my shit through registered mail.

normally the compressed weed lasts quite long but maybe thats coz its so weak - i live in s.africa and i get my shit direct from lesotho, malawi, zimbabwe and zambia (ALL THE GOOD SHIT!!! HELL YEAH!!!) the stuff is strong as f**k and doesn't keep for longer than 2 weeks. after that it just dries up and crumbles into powder.

I guess different types of weed have different strengths and the stronger the weed is the shorter the shelf-life is.