Socci .... sorry, have only just seen your question. I really think you'd need to go to a shop and try them on - I'm pretty much a 10 in the high street although wear an 8 in Sandwich, my Sevens are a 26" waist which is, I think, an 8. However, the girl in the shop got me to buy them tighter than I would have chosen ... but she was right as the next size up was way too baggy around the groin (and I guess it shows that I've been buying my Gap jeans a size too big recently ... which is why they're saggy around my butt!)
Hope this is of some help.