How do I run a disciplinary when we have no HR department?

The answer Just Ask, HR gives

Detailed Dan — Structured and thorough — every step in order.

You don't need an HR department — you need to follow the ACAS Code of Practice. Tribunals judge small employers against the same fairness principles, applied proportionately to your size. Here is the full sequence:

1. Investigate first. Gather facts before any hearing: statements from witnesses, records, the customer complaints. In a small business the investigator and the hearing manager should ideally be different people — if that's impossible, keep the roles clearly separated in writing.
2. Write the invitation letter. It must state: the specific allegations (dates, incidents), the evidence attached, that the meeting is a disciplinary hearing, the possible outcome (e.g. written warning or dismissal if gross misconduct), and their right to be accompanied by a colleague or trade union representative. Give at least 2–3 working days' notice.
3. Hold the hearing. Put the allegations, show the evidence, and let them respond fully. Take notes — a second person as note-taker transforms the quality of your record. Adjourn before deciding; never announce the outcome in the meeting.
4. Decide proportionately. First offence of rudeness is usually a first written warning, not dismissal. Check what you've done in similar past cases — consistency is half of fairness.
5. Confirm in writing with the reason, the warning's duration (typically 6–12 months), what improvement is required, and the right of appeal.
6. Hear any appeal — by someone not previously involved if at all possible; if you're the owner and did the hearing, a director or external HR consultant can hear it.

An unfair procedure can make an otherwise justified dismissal unfair, and a tribunal can uplift compensation by up to 25% for ignoring the ACAS Code. The process is the protection.