Where an employer recognises one union can a worker ask to be accompanied by an official of another union at a disciplinary or grievance hearing?

Yes. There is no requirement in s.10 of the Employment Relations Act 1999 that a trade union official who accompanies a worker at a disciplinary or grievance hearing must be from a recognised trade union. The official must simply be either employed by a trade union of which he or she is an official, or a lay official of a trade union whom the union has reasonably certified in writing as having experience of, or as having received training in, acting as a worker's companion at disciplinary or grievance hearings. Certification may take the form of a card or letter. If the worker is a member of a non-recognised trade union, the worker may therefore ask to be accompanied by a trade union official from that trade union. Indeed, there is no requirement in the legislation for the worker to be a member of the trade union at all to be accompanied by a trade union official, although in practice trade unions rarely support workers who are not their members.