Thx everyone for your great advice. Had a look on Defra's website, it's quite clear that when a person purchases a horse, they have 30 days to update ownership details. I guess therefore, if someone is selling a horse that doesn't have their name on the passport, is not the legal owner.
Thinking of buying a horse, however, have come across a discrepancy in what seller is telling me & not sure if I'm being naive.
Having checked the horse out on breed society website, the database is telling me horse is owned by a different name to that of the seller, selling the horse...