Fab reading Anna, please add it to Angie's next Link Love to spread the word.
I've learnt since joining here not to shop for SIZE but for FIT. Because, in my case, I am sometimes two sizes apart. My tall narrow-shouldered pearfect frame requires UK size 14 tops in some stores, while the bottoms can go as far as 18!
Here are just a few of my examples that used to be frustrating, but I've now learnt my lesson (written about it here):
I personally have noticed that trendier shops tend to run very, very small for me - they obviously cater for teeny-weeny trendy girls regardless of boasting with ranges that should appeal to some niche markets (I pretty much had to stay away from Topshop on bad days because I couldn't come anywhere near their size 14 although I was desperate for long jeans - they have Tall range, but nothing that ever fitted me).
Niche market shop, like the one where I buy a lot of tall clothes, Long Tall Sally, is ridiculous in another direction. I actually have some tops that are XS and S! This is so obviously geared towards giving some tall women who might feel gigantic a small sense that they too are dainty and fragile or whatever (ok, this is frustration of several re-orderings speaking, but when you start with size L, but end up with XS it's a lot of parcels and runs to the post office - only to discover that some other type of clothing, e.g. dresses, item actually needs to be ordered in L or XL).
Britan's favorite department store M&S is also not 100% reliable - I have skirts in 14 and 16, they fit the same, but the difference comes from the actual style. Also, they must be changing the patterns a bit to fit their wider market into the smaller sizes, because I've actually had some small sizes on because the ones I could normally pick without trying were a bit big (my weight had not gone down).
Also, I've come across the reviews which mention that the same model in different colors fits differently - this would be the case when e.g. certain top in one color is made in one country, while different color is made in another country. Go figure.
The bottom line - try it on and forget the numbers, size isn't everything.