the Mirror. Which you can't always, but they did get a quote from an actual BBC spokesperson: “The decision for Peter Capaldi to have a Scottish accent as the Doctor was a creative decision and it was not part of his contract.”
(Well, he's quite capable of playing parts without the accent, so a more Southern one would have been a possibility. The comment seems to have been sparked by an earlier rumour that it was genuinely written into his contract, so this is quality journalism for the Mirror.)
They spend most of the article talking about how disappointed David Tennant must be (he wasn't allowed to) and quote a "show insider". “But Peter is best at acting in a Scottish accent and he was delighted when he found out he could play the part with his own voice. It gives his character a stronger identity as well.” Quite possibly, they tried it out both ways and decided that the Scottish accent sounded better.