At the end of a three-day trade show, a marketing director told me the worst part hadn't been the stand. It was watching her team put up with one buyer's behaviour for three days because he was a big account.

She hadn't known what she was allowed to say to him, and nobody above her had ever told her.

What changes on 30 October

From 30 October, that gap becomes the employer's problem in law, and not just an uncomfortable few days at an exhibition. Acas sets this out on its page about third party harassment under the Employment Rights Act. Right now, an employer can be liable if one worker harasses another, unless they took all reasonable steps to prevent it.

After 30 October the same test applies when the person responsible doesn't work for you at all.

And it's a big widening. The person could be a client, a customer, a contractor or somebody you've never met. You could be on the hook for how your workers were treated, unless you'd taken all reasonable steps.

Who counts as a third party for a marketing team?

Acas says a third party could be anyone a worker deals with through their job, including people who don't work for you. Its examples include customers, clients, self-employed consultants and contractors, and people at conferences or events. It also says the harassment can happen online as well as in person.

Look at a marketing team's month and the list fills up quickly.

Think about the agency partners and the freelancers. The clients on a launch. A trade show with a few hundred strangers. And a brand account where anyone can send anything to whoever runs it.

Any of those could now count.

What counts as all reasonable steps?

Acas says what's reasonable will likely depend on the organisation, including its size, its sector and the kind of work people do. But the test is what's possible with the resources you have. Some steps won't be possible for every organisation.

The steps it lists are practical. Assess the risk of third party harassment regularly, keep a record of incidents and what was done, have a written policy, and train people in things like staff safety and de-escalation. Check that your client, customer and supplier contracts include the relevant parts of your policy. And tell customers what the consequences are if they harass your staff.

Acas also gives an example of an employer getting it wrong.

A shop owner takes on two new workers, who report threats from customers, and in one case racist abuse. The owner tells them it's all part of the job and there are plenty of people who'd take their place. Acas says that employer is very unlikely to have taken all reasonable steps.

What went wrong there was the reaction, not a missing document.

The step that depends on who runs the team

Most of that list can be written down in an afternoon. But the step that decides whether any of it works is what happens when somebody on the team says a client made them uncomfortable. In a marketing team, that conversation goes to whoever leads it.

If that person treats it as part of the job, the policy is decoration. A leader who takes it seriously, deals with it quietly and will have an awkward word with a big client gives the policy a chance of working. And you can't tell which kind of person you're hiring from a CV.

That's why I'd put it into the interview whenever you're hiring a head of marketing or a marketing director.

What to ask a marketing leader at interview

Start by asking who their team deals with that doesn't work for the company. A good candidate will have the list ready. The agencies, the freelancers, the people at events, whoever's in the comments. But a vague answer tells you they haven't thought about it.

Then ask what they did the last time somebody on their team came to them about a client or a supplier.

You want a real example, what they did next, and whether that team member would say they felt backed. A rehearsed answer is easy to spot, because a real one has awkward detail in it.

And ask what they'd do if the person involved was their biggest client. Some will say they'd handle it quietly and keep the account happy. Or they'll say they'd look after the team member first and sort out the account afterwards. You'll know which one you want running your team.

I'd also ask about the brand's social accounts.

Acas says online harassment counts, so somebody in the team is usually reading the worst messages the brand gets. The leader should know who that is. And they should know what that person has been told to do with those messages.

What to do before 30 October

Spend an hour listing who your marketing team actually deals with outside the company, and write down who each person should go to if something goes wrong. Acas suggests the same: consider which third parties your workers are in contact with, and make sure they know who to contact and how to raise a concern.

Then look at the paperwork. Agency agreements, freelancer terms and event bookings are the obvious ones. Acas says to make sure they include the relevant parts of your policy. And start a simple record of incidents too, since Acas lists monitoring and recording them as one of the steps.

It isn't only about third parties, either.

The duty to take all reasonable steps to prevent sexual harassment starts on the same day. Liability for third parties arrives alongside it. In a YouGov poll of 1,050 senior decision makers in Great Britain, commissioned by Acas in the spring, 41% weren't aware of the sexual harassment change.

If you're one of them, the third party question is a good reason to look at the whole thing at once.

And if you're recruiting a head of marketing or a marketing director at the moment, send us the role when you're ready. We'll talk through what the person needs to be able to do, including how to test for this at interview. Our guide to hiring a head of marketing is a good place to start, and you can see what our marketing recruitment team covers.