weights are consistently 0-100g, 101-250g, 251-500g, 501-750g, 751-1kg etc.
Price difference is letter, large letter and packet. I was told that a video tape is a good guide to follow as max width for large letter.
Letter - c5+ or under, up to 100g less than 5mm thick 240x165mm
Large Letter - B4 or under - up to 750g and less than 25mm thick 250x353mm
Packet - More than 353mm long or more than 250mm wide or more than 25mm thick. If one of the dimensions exceeds this or the weight exceeds 750g, the item is classed as a packet.
Recorded is always 72p on top of normal postage.
Basically Craig if it fits through the slot for large letter then you can be charged L/L, if you unsure then send it as pkt, cos better that you pay a bit more than recipient receiving it with no postage and having to pay that and admin. it also loses its priority in the system too
Its stupid though, cos my worry dolls with padded envelope, bubble wrap, packing slip and fridge magnet still dont reach 30g but have to charge packet money for them, whereas a much bigger, sometimes heavier item comes in for much less on p&p.
You can download the postal charges from Royal Mail - I have mine laminated and refer to them often with new listings. My son gets dvds from HMV and they come in nice just about packet width boxes, so if an item wont go in there comfortably, I know its a packet.
Hope my waffling helps just a little bit!