(d) Procedural fairnessPitt (9th edn) 299, [8.045]
A Failure to Follow Procedures May Affect the Fairness of the Dismissal
(i) Good Practice and the Law
ACAS Code of Practice on Disciplinary and Grievance Procedures 20152 issued by ACAS under TULRCA 1992, s 199.
Section 207A, TULRCA 1992 – adjustment of award, up or down.
Any award can be increased by 25% where employer unreasonably failed to comply with the Code. Any award can be decreased by 25% where employee unreasonably failed to comply with the Code
(iv) The Rule in Polkey
Polkey v AE Dayton Services Ltd  AC 344;  ICR 142 (HL)
Lord MacKay (for the House) p 363: [Relying on Devis v Atkins above]
In my opinion, therefore, the additional reasons given by the Court of Appeal in the present case for supporting the [‘no difference’] principle involve an impermissible reliance upon matters not known to the employers before the dismissal and a confusion between unreasonable conduct in reaching the conclusion to dismiss, which is a necessary ingredient of an unfair dismissal, and injustice to the employee which is not a necessary ingredient of an unfair dismissal, although its absence will be important in relation to a compensatory award.
Sillifant v Powell Duffryn Timber Ltd  IRLR 91 (EAT), Browne-Wilkinson J. pp 93 & 96:
‘…the [‘no difference’] principle appears to have become established in practice without it being appreciated that it represented a fundamental departure from both basic principle and the earlier decisions. If we felt able to do so we would hold that it is wrong in principle and undesirable in its practical effect. It introduces just that confusion which Devis v Atkins was concerned to avoid between the fairness of the dismissal (which depends solely upon the reasonableness of the employer’s conduct) and the compensation payable to the employee (which takes into account the conduct of the employee whether known to the employer or not).’ …
‘There is no need for an ‘all or nothing’ decision. If the industrial tribunal thinks there is a doubt whether or not the employee would have been dismissed, this element can be reflected by reducing the normal amount of compensation by a percentage representing the chance that the employee would still have lost his employment.’
See Vokes v Bear  ICR 1 (NIRC) (=EAT)
2 < http://www.acas.org.uk/media/pdf/f/m/Acas-Code-of-Practice-1-on-disciplinary-and-grievance-procedures.pdf > accessed 27 Oct 2015.