Hi Lolas Angel. Its tough isnt it! I am just like you, always wondering how much to give my two girls aged 11 and 7. We also live in Surrey, and I give my 11 year old 10 pounds per week, and our 7 year old 5 pounds. For this, they must complete all homework before they get it on a Saturday, and their weekly magazines come out of this money (the magazines often cost 3.00 which takes up a large chunk).
We pay for sweeties as treats, and they enjoy thinking about what they are going to spend it on. Like you, I try and get them to do chores too. My elder daughter now has a vending machine at school for drinks and things, so her new job is to unload the dishwasher in the morning and feed the cats their dried food which she does without fail. I would have thought that your oldest boy had maybe cause for some increase, but I am guessing that if he goes swimming, cinema, bowling etc that you give him money for this on top of his pocket money?
A typical example is the really expensive sweets at the cinema, they ask 3 pounds for a bag of Maltesers, so that would be his allowance gone in one go!. I get my two girls to know the value by going to the supermarket and getting them to pick up the sweets there beforehand, where they cost under a pound for the same thing