I'm just posting this in case it helps anyone building as a hobby and on a budget, or doing a one-off re-fret. Yes I know, it's not as good as the real thing, and it will no doubt offend some purists and stewmacolytes, but anyway...
You can buy from various sources a file built into a block and angled (usually at 35 deg) for filing an angle onto fret ends; you slide the block up and down the neck, and the file dresses back the frets and (if you wish) the sharp corner of the fingerboard. If you don't like the price or can't stand the wait for delivery, various youtube videos show you how to make one; but they generally need a table saw to cut the angled slot, and we don't all have one of those.
I managed to make do by knocking the handle off a triangular file and clamping it (the file, not the handle!) to a suitable bit of flat-faced timber using two thin offcuts of ply, the edges of which I'd sanded to an angle to grip the file (see pic).
The main piece of thin ply is glued, the smaller one screwed; so you can easily have your file back when you're done. Here's how it's used (you can see the file's rat-tail beside the fretboard):
The angle of cut is around 30 degrees, which is near enough for me, and the tool was free since I had the file and the offcuts handy. Here's the result:
Sorry for the crappy photos (telephone in badly-lit garage!)
Hope it helps someone out.