The relevant rule here is rule 41. The Tribunal may give a decision orally, and ordinarily do.
During the coronavirus pandemic the panel can decide whether to announce the decision, and either provide a time for the oral decision, or email the decision to the representative and the MHAA: Mental Health Tribunal, 'Video Hearing Guidance for Representatives in Mental Health Tribunals' (11/9/20).
Subject to the rules on non-disclosure of documents/information, written reasons must be sent within 7 days (3 working days for s2 hearings). The secretariat does not send written decisions directly to patients if they have legal representation.
A case involving the immigration tribunal (whose rules are materially the same) stated that if the tribunal gives an oral decision then that is the decision of the tribunal, which cannot be revised or revoked; however, if the written decision is inconsistent then both decisions stand but cannot be enforced until sorted out on appeal (PAA v SSHDM).