Hi, welcome to TMA

Whenever anybody mentions unrecorded stone circles in Wales I immediately think of Eisteddfod circles put up in recent years to comemorate those events. Is this the circle you saw?-

http://myweb.tiscali.co.uk/celynog/modern_circles_3.htm

(second one on page) Trevine is also spelled as Trefin, plus your description seems to match. If so then sorry it's not an old one and hence not covered in the books or on here either, but it certainly looks like a nice place.

Cheers-
Chris