Yeah, I've always wondered that - I always assumed it was some EU thing somewhere, but TARDIS wiki would have that all right if it were. (I peeked behind the scenes at the comments on the AO3 tag to see if someone had left a source for using that, but nothing. My guess would be that maybe at the time it came in, that's how it was on TARDIS Wiki, if there have been deletions since?)

Pronounciation would be sort of "Devwyth/Devouwyth" except probably not exactly quite like that, but near enough. It's not really a typical Welsh surname, either, especially not for South Wales.