This one should have presented no great problems for you. If it did, you need to go back to your first lesson in swallow ID; or at least look in a book and learn. Here we have a black-and-white looking bird, with a beak full of wet mud. The underparts are a clean, pure white from the throat to the undertail coverts. Even the legs are feathered white, and incidentally, are far too long for a swift species (which have tiny legs). The upperparts appear black or very dark, glossy blue-black. But the white of the underparts seems to wrap around to the rump (just behind the wings); we only have one hirundine with a white rump. The underwings are dark. All the elements are there to identify this as a House Martin, gathering mud for a wall-mounted mud nest.
KEY FEATURES Clean white over whole of underparts, even ‘trousers’ White rump Dark underwings Blue-black head, back and upperwing