The per diem is £75 per day, regardless of how long the writing session lasts.
Universal Music Group UK labels and Warner Music Group UK labels pay this rate directly. Sony Music Group UK label per diems are paid through the Sony Music Songwriter Support Fund, administered by The Ivors Academy.
Other labels may offer different arrangements and you should ask the record company representative or the person who invited you to the session what support they offer and how to access it.