I buy for quite a few lads in this age range. My go-to gifts this year for is Superdry. You can get a bag or hoodie from the eBay Superdry outlet easily for that budget.Official outlet here. I got a Superdry watch for DN(13) one year for £25 and it was apparently his favourite present. If you want to top it up to £30, he's the right age for the Beanboozled game. You can get refill packs in Home Bargains for £1.
Thanks very much for all of the suggestions. Will run some of these ideas past my sister! Don't see dnephew often enough to know what he is into, and she isn't always able to come up with ideas either, so it's good to be armed with a list of possibilities.
I agree a voucher for 'game' or gift card for Xbox or 'steam' card for gamers, most boys that age are playing some sort of games and always need the latest game or 'in game' credit for upgrades and in game purchases.
I agree with the poster up thread who suggested beanboozled, always a big hit! (Or those chocolate roulette games with hidden chilli)