Investigations - Right to Representation?

There seems to be some conflicting advice out there in relation to conducting investigations.

At present as a company we inform the employee in advance of their requirement to attend an investigation meeting giving a minimum of 3 days notice, and offer them right to representation. We make it clear in the letter that it is just an investigation meeting and that it is not a disciplinary hearing.

Is this the correct process? The new ACAS guidelines don't seem to support this, but I'm nervous about not offering representation in meetings such as this as it is often of some comfort to the employee to have someone with them.

I would be interested to know what everyone else does in this situation.

Thanks