Looks far too dark for a sand martin, upperparts are usually alot lighter in juvenile sands wheras this one has all the hall marks of a house martin as its upperparts, at least those visible, are very dark indeed.
Sand martins are actually much more brown, sandy, especially juveniles, excuse the obvious pun. I can see the vague band across the chest but have seen juvenile house martins with these bands, literally having seen them fledge staright out of a house martin nest. Sand martin bands are generally very very noticeable and quite thick, often with a dark line running vertically down the chest also in juveniles. The band on this bird for me is too vague and the head is too dark and does not show enough brown.
It is also possible that it cold be a sand/house cross, which is not all that uncommon as is a house/swallow cross.