"Popchyk has pointed out on the other thread that it would have been Norfolk rather than West Yorkshire police that dealt with this. So can we still trust Harrup?"
Not sure about that. The warning can be issued over the phone.
I think Harrop is speaking the truth here.
Note that it's not a finding of wrongdoing, nor an admission of the same, they are literally warning him that if he carries on with the behaviour then it might be an offence.
The police just want to close the case and move on. And as such it is closed without a finding of wrongdoing as such, at this point.