Juvenile Barn Swallows are very variable in their plumage shortly after fledging. These may seem atypical birds (compared to a stylised example in a field guide) but it is not unusual to have variable chest/breast markings (and the same applies to the white patches in the scapulars and mantle). Barn Swallows they most definitely are.
Thank you Colin. I shall take them out of the query category.