I found an occupied mine on Greater Bird's-foot Trefoil at Lanlay Meadows yesterday. There are two species of Trifurcula which mine bird's-foot trefoils: T. cryptella and T. eurema, both of which are scarce (the latter not recorded in Glamorgan). T. eurema is apparently a downland species which mines Common Bird's-foot Trefoil whereas T. cryptella occurs in more wooded habitats and mines either Common or Greater Bird's-foot Trefoil. T. cryptella apparently has blackish frass whereas that of T. eurema is brownish.
I think this is T. cryptella based on the blackish frass and foodplant. I'm hoping Dave can confirm as I think my chances of rearing the adult are slim!