"Exceptionally, a student who presents themselves for an assessment or submits coursework may, at the discretion of the Chair of the relevant Assessment Sub Board, be retrospectively granted an authorised absence from that assessment provided that they submit evidence of mitigating circumstances normally no more than seven calendar days after the date of the assessment or submission deadline. Supporting evidence should be submitted with the form provided for this purpose or within 21 calendar days of the date of the affected assessment." (T43.4).
This means that you can submit a MC request up to 7 days after your assessment or submission deadline, but you will be asked to provide a good reason as to why you did not follow the normal mitigating circumstances procedure before you presented yourself for an examination or submitted coursework (T43.6).