I'm a bit behind times here as my experience is out of date, but assuming this is purely pocket money for him, to buy sweets/toys/magazines out of and you aren't expecting him to buy clothes/bus pass/phone credit/toiletries out of it, I would have thought £5-£10 a week is about right.
Personally I don't like the idea of paying children for 'everyday' chores like laying the table, helping with washing up, laundry, tidying their rooms - to me those are kind of the basics/part and parcel of family life you don't particularly get 'rewarded' for doing, like doing your homework or brushing your teeth. Plus if yours were anything like mine at that age, they get so lazy that the threat of docking pocket money didn't do much to motivate them to get up and do their housework, they didn't really value the £5 worth of sweets they'd get at the weekend highly enough to bother to do boring chores on a Monday without being forced to, in fact it would become a perverse disincentive to get me or their siblings to do their chores in 'exchange' for their pretty measly pocket money (not a minimum wage hourly rate anyway
) so we gave up on that and pocket money was given regardless of chore completion (compliance there was punished/rewarded in other ways)! I did occasionally dock pocket money as a 'punishment' but only where there was a direct monetary link, e.g. the one who was careless and perpetually lost school uniform items had money deducted towards the amounts I was shelling out for replacements. This seemed more logical and effective, for us anyway.
So I think if you would like him to have pocket money (and I think it is good for them to have it, to start to learn rudimentary budgeting, saving and delayed gratification!), give a token amount to him unconditionally (unless as above he is repaying lost or broken items), and find another way to insist he does chores. You could always offer a small amount for 'bigger'/non everyday jobs like £5 to wash the car, £10 to weed the garden etc, so he can 'earn' more for discretionary effort?