If a transfer direction for a section 47/49 is dated for example 16th September 2020, but the patient was not admitted to hospital until a week later, does the section date begin on the date of the transfer direction or the date of admission to hospital?
It’s the date of the direction; see Schedule 1 of the MHA.
I can’t find anything in schedule 1 directly about the start date of s47/49 detention, but I think your conclusion must be right.
Schedule 1, Part 1, para 6 applies s20 (Duration of authority) to unrestricted Part 3 patients, modifying it so that detention begins on “the date of the relevant order or direction under Part III of this Act”. Section 20 does not apply to restricted patients, but it would be odd not to apply the same principle.
Also, s70 provides for tribunal eligibility periods which are based on “the date of the relevant … transfer direction”.