Thank you all for your helpful comments, it has been quite enlightening.
Just to clarify my own situation, which I deliberately avoided initially, I am not yet in Thailand so cannot discuss with a visa agent or local IO. I'm also not living in my home country so cannot apply through the embassy or consulate there.
I had read some time ago that the method outlined was official but can't find where I saw that, therefore posted my query in the event that there were people with knowledge of this.
Anyway, thank you again and keep the info coming. Perhaps it may help others for future applications also.
That's not correct for everyone. It depends which country you are from. The pension department of your country specifies who can be a witness. Thus will vary from country to country of origin, and is normally outlined on the letter attached to the Proof of Life certicate( which in New Zealands case comes every 6 months).