In Yorkshire Housing v Swanson, the EAT has held that a dismissal is automatically unfair when the employer delays unreasonably in following the statutory dismissal procedure.
With "some unease" (para 67), the EAT stated that the delay provisions, despite being in a different Part of Schedule 1 to the Employment Act 2002 to the standard dismissal procedure, were mandatory rules that had to be followed - breach of which led to a finding of automatic unfair dismissals. The EAT acknowledged the difficulty construing unreasonable delay as falling within "non-completion" (para 64), since very often the procedures would be completed - albeit late - but held for policy reasons (para 69) that there should be a finding of automatic unfair dismissal.
On the facts, the employer had delayed for five months between holding the disciplinary meeting and writing the letter dismissing the Claimant. That was (unsurprisingly) held to be unreasonable delay.
Thanks to Sally Cowen of Cloisters, who appeared for the Claimant, for telling me about this case.