Section 10.1(a) of the Employment Relations Act 1999 states:

Right to be accompanied

(1) This section applies where a worker—

(a) is required or invited by his employer to attend a disciplinary or grievance hearing, and

(b) reasonably requests to be accompanied at the hearing.

(2) Where this section applies the employer must permit the worker to be accompanied at the hearing by a single companion who—

(a) is chosen by the worker and is within subsection (3),

(b) is to be permitted to address the hearing (but not to answer questions on behalf of the worker), and

(c) is to be permitted to confer with the worker during the hearing.

(3) A person is within this subsection if he is—

(a) employed by a trade union of which he is an official within the meaning of sections 1 and 119 of the Trade Union and Labour Relations (Consolidation) Act 1992,

(b) an official of a trade union (within that meaning) 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, or

(c) another of the employer’s workers.

(4) If—

(a) a worker has a right under this section to be accompanied at a hearing,

(b) his chosen companion will not be available at the time proposed for the hearing by the employer, and

(c) the worker proposes an alternative time which satisfies subsection (5),

the employer must postpone the hearing to the time proposed by the worker.

(5) An alternative time must—

(a) be reasonable, and

(b) fall before the end of the period of five working days beginning with the first working day after the day proposed by the employer.

(6) An employer shall permit a worker to take time off during working hours for the purpose of accompanying another of the employer’s workers in accordance with a request under subsection (1)(b).

(7) Sections 168(3) and (4), 169 and 171 to 173 of the [1992 c. 52.] Trade Union and Labour Relations (Consolidation) Act 1992 (time off for carrying out trade union duties) shall apply in relation to subsection (6) above as they apply in relation to section 168(1) of that Act.

Details here.