Actually, it seems like they are mostly sequential, not random, and there's no check digit. This is supported by the two tickets I booked last night within some twenty minutes of each other, one with BA, the second with AB. Both record locators start with the same three digits and the fourth is only one apart – though of course that may just be coincidence.

This is the second time I post this talk this week, but these security researchers go into quite a bit of detail about booking references and their flaws: