My experience with joules is that sizes are fairly spot on- so when I'm a 12 - their 12's are spot on. If I'm carrying my spare tyre, I need the 14.
Therefore I'd say go for the 14 if you are on the larger side of 12.
Plus bound to be a 15% code with free P&P and returns if you have a look about - that way it can always go back if not right