Before buying: match horse to passport (description, microchip) and ensure the seller hands you the passport at sale. [gov.uk]

Within 30 days of purchase: contact the issuing PIO on the passport; submit their change‑of‑ownership form, passport, and fee. Use tracked post.

Microchip check: if chip not recorded or absent, arrange vet + update passport. [gov.uk]

Digital Stable: claim your horse and check the CED shows you as owner; correct discrepancies online. [equineregister.co.uk]

Keep the passport with the horse (at the yard and whenever it travels). [gov.uk]