According to Elizabeth Marriott, secretary of the Tristan da Cunha Association, this is an official legal tender set:

(image from Joel Anderson)
The set is available from Joel's site:
LINK: www.joelscoins.com
Welcome to my numismatic blog. Find information about releases of new world coin types and varieties.
Please feel free to email me at jo7lan@gmail.com for corrections, enquiries, suggestions or whatever.