I have a print out of an extract of the liber extra 1234, 4.15.2 on impotence in marriage. it starts 'You took a wife and you had her for some time, a month, three months, or at most after a year. Now for the first time you have said that you are of a frigid nature so that you cannot copulate with her or with anyone else.' I can not find where the original case comes from. It is in reference to Brocard XIX which I assumed meant the Gratian's decorum, but again cant find it referenced. Is there any gender of mediaeval historians help a gal out?
What you're looking for is the Corrector of the late 10th/early 11th century writer Burchard of Worms, book 19 (xix) of his collection of canon law, Collectarium Canonum, also known as his Decretum (it's a predecessor to Gratian's Decretum, which is a 12th century work). "Brocard" is a term derived from the Latinization of Burchard's name that came to denote a legal maxim or tenet. Burchard's Corrector is a penetential, that is, a work specifically designed to assist in the identification and definition of sin so that appropriate penance could be assigned.
If you'd like to get at Burchard's original text, it's available as part of the Patrologia Latina (PL) in volume 140, which is digitized online here: https://archive.org/details/patrologiaecurs17unkngoog/page/n486/mode/2up
Book 19 starts on p. 943. The passage you are looking for is on p. 967, under the heading De discidio conjugii:
Accepisti mulierem, et habuisti eam aliquod tempus, mensem, aut tres, aut postremo aununi, et tunc primum dixisti te esse frigidse naturse, ita ut non potuisses coire cum ea nec cum alia aliqua : et si illa quse uxor tua esse debuit, eadem affirmat quee tn dicis : et si probari potest per rectum judicium, ita esse ut dicitis, separari potestis, tamen ea ratione ut, si tu post aliam acceperis, reus perjurii dijudiceris, et iterum, post peractam pcenitentiam, priora connubia renovare debetis.