Just had a response from Thai Airways along with an attached copy of my booking which shows my UA FF number on it. "Thank you for contacting us.
The attachment is your booking when you travelled with us. You can see line 19 & 20 where your UNITED airlines member numbers were added into you booking
There is no record of any THAI airways member number being entered, therefore we are unable to remove something that does not exist. I can only suggest that you go back to UNITED with this information.
Kind regards
Chris"
I have a funny feeling that UA are just going to stonewall me on this, even with the above statement. I will persist though - seems an insane situation to me.