I am a size 14 in jeans, have been for years and always got this size in River Island, perfect fit.
I go in Thursday, pick up a pair of the skinny flared variety, my favourites. I can't believe how small a 14 is so pick up a 16. Too small. I fitted into a size 18 
I have not suddenly piled on weight and am in denial! I still have my older RI size 14 jeans here and they fit me perfectly. An 18!? What actually happened to their jeans sizes? More than depressed at the size 18 tag, had to get them as they are the best cut and jean to suit me, needed a black, black pair as my others were so old they were only fit for the bin. God awful sizing frankly. How to utterly depress a customer in one easy step.