Please help me find the perfect word to shut down HR

That sounds very much like how HMRC are acting nowadays.

"We want this information and we want it within 2 weeks."

We bust a gut to respond and send the reply within the deadline.

Four months later:

"We want more information and we want it within 2 weeks."

I feel your pain!

Obviously not getting the full story here but maybe asking for a chat would be better than ranting on here.

In order to avoid any confusion or suspicion of impropriety and inherent breach of Professional Standards and Statutory requirements, I must therefore restate my position that under no circumstances is any communication in my name to be forwarded to the independent investigator without my prior approval.

My team would love that.

To what “Professional Standards and Statutory requirements” are you referring?

Unacceptably overbearing attitude

So you’re a run of the mill witness (not an expert). Are you holding things up?

whilst I don’t think any rude emails are acceptable, you have no idea how frustrating it is to try and drive these processes when everyone else thinks they know everything better. <pours wine>

'I do not appreciate the tone of your emails. I am not your subordinate or an employee of X company, I am helping you purely on a voluntary basis and I would like this reflected in your future communications or I will be reconsidering my involvement'.

That might be a bit strong though.

I would go with this. Can you speak to someone on the phone also, in case you're getting automated messages.

As you are aware I have agreed to provide x services on a purely voluntary basis. As such I would appreciate for HR colleagues to use a more appropriate tone in what is essentially a collaborative arrangement rather than me feeling that they are dictating the extent of my contribution and involvement.

Is this NHS- an SI investigation? If so, then managers of other teams are obliged to participate on a rota and would be in your JD.
If not- sorry!

Just remind them that you're not an employee and are helping on a voluntary basis, and would therefore appreciate a mutually respectful interaction. If that's not possible then you will be unable to assist further.

They have completely missed the point that I am not one of their employees and am doing them a favour. I owe them nothing.

Then that is exactly what you should tell them. Don't say 'bully.' It comes over as a bit infantile and persecution complex-y.

Say something like 'I would like to remind your HR department that while I am happy to co-operate with their investigation, I am under no obligation to do so. I am not a company employee and I would therefore appreciate a change of tone in their correspondence to me.

But you need to follow the organisation’s HR processes and policies for it to be a fair investigation. If you don’t want to do that, then you’re in fact not happy to help, and shouldn’t volunteer. Of course they’re going to give you direction, they’re responsible for ensuring compliance with their processes. Otherwise they’ll be responsible when there’s an ET and the company receives a large fine for not following its own process…

‘Thank you for your email dated XXX.

As I am sure you can appreciate, in situations like this, it is important to me that the external organisation has all of the information I have so that a thorough, non biased and transparent investigation can be carried out.

Therefore, as an external and voluntary agent in the process I feel it’s important for me to be clear that;

1. I will be sending XYZ to both yourselves and the external provider separately to ensure complete transparency.
2. I will do this by X date in Y format.’

This.

Please bear in mind that I don't work for you, so your deadlines are not my deadlines.

@FromTheBubblegumTree - just out of interest, is this to do broadly with the education sector? Please feel free not to say (or PM me), but I’ve had something v similar recently. Feel for you. It’s a lot of responsibility on your shoulders and there’s some murky practices. 💐

"I am writing to clarify my involvement in assisting with your investigation. As I am not an employee of [Company’s Name], my participation is entirely voluntary, and I am providing information solely to aid your team in reaching an informed outcome.
*
While I am committed to supporting your efforts, I am not bound by [Company’s Name]’s internal policies or procedures. I respectfully request that my voluntary role is acknowledged, and any requests for information are made with this understanding in mind.*"

It's hard to know without seeing the email, but is it the case that you have to agree to comply with their processes if you are to be involved? If so, volunteer or not, the statement is not unreasonable, though the tone does not sound ideal.
Do you feel you have to say something?

I know the convo has moved on but I think 'hector' is the word you were originally looking for

Ah - that is slightly different. In which case, I would simply respond saying that you will respond to the third party investigating officer directly; you have no intention of entering into any other correspondence with them; and to cease contacting you or else you will block them. Simple as. I wouldn't bother being "polite" about it, just firm. Your involvement is nothing to do with them, and purely a matter for the third party investigator.

There are ways to say that and then there are ways to say that.

@FromTheBubblegumTree why the hell are you helping and hr department if you do not work for the company??? this doesnt make sense!