I think the first thing to do is write a letter to the credit reference agency concerned, explaining everything and if possible, send proof that you are not Daniel Kirby. They could, of course, be suspicious that you're using an alias to obtain credit by deception and I suppose you do have to see it from their point of view: A guy with almost the same name as you, even right down to the same date of birth. I think it might also be wise for you to write a letter to the NatWest bank explaining the situation. Obviously they won't give out details of another person's account but surely they must have Daniel Kirby's signature on file, which could be a step in the right direction of proving you and he are two completely different people.
Alternatively, you could have a word with the Citizen's Advice Bureau, but personally, I'd get those letters written and sent off to the credit reference agency and the NatWest. Remember, the longer you leave it, the more suspicious it looks.