Following from Leo van de Pas in a posting to soc.genealogy.medieval newsgroup, 23 Feb 1999:
I found the following about Joan bastard of England, after her death she was buried at Llanfaes but, at the dissolution of the monastery, her coffin was removed. In the 19th century it was discovered that it was being used as a horse trough. Rescued from this use, it was placed in Baron Hill Park, near Beaumaris in Anglesey. The most recent location is given as the Church of St. Mary and St. Nicholas in Beaumaris.