Opening hours for "regulated" stations (which basically means those where the Ticket Office is run by a TOC) are defined in Schedule 17 of the Ticketing and Settlement Agreement.
Compliance is mostly handled by "Compliance and Audit" (or similar) departments within the TOCs themselves, though Rail Settlement Plan can also conduct audits themselves. In the case of a sustained major failing, the TOC could potentially have their licence to act as a retailer terminated.
Looking at Schedule 17, the relevant hours for Brighton are 0345 to 0145 every day, with tickets requiring reservations not available in the morning peak (0600 to 0815) Monday to Friday.
