Sebastian is the current first six pupil at 36 Crime.

Prior to coming to the bar he worked for four years at the Law Commission and the Attorney General's Office. At the Law Commission Sebastian was the lead lawyer on the Law Commission's Sentencing Code project, instructing Parliamentary Counsel on the drafting of the Bill and giving evidence to the Joint Committee during the Code's passage through Parliament. At the Attorney General's Office he worked on a range of topics including unduly lenient sentences, contempt of court, applications for consent to prosecute (including consents for explosive substances, year and a day prosecutions, international conspiracies and Armed Forces Act offences) and applications for second inquests. Notably he lead on an application to amend the Thompson and Venables injunction and worked closely with the Civil Procedure Rules Committee on the redrafting of Part 81 of the Civil Procedure Rules (Contempt).

During this time Sebastian also taught criminal law at the London School of Economics. He continues to teach law at the University of Cambridge. He is also the author of a number of practitioner-texts and journal articles, and his academic work has previously been cited by the Court of Appeal (Criminal Division).• Faculty of Law, University of Cambridge
• Gray's Inn