Merdinus

In Welsh, where he started, the wizard in the Arthur stories is called Myrddin. In English we say Merlin, which comes from his Latin name, Merlinus. The Latin name seems to have been made up by Geoffrey of Monmouth (yes, him again). Now, there's no sound in Latin corresponding to Welsh "dd", but generally you'd represent it with a similar sound, like D. So why on earth did Geoffrey change it to an L?

Well, I read something today (and now I can't find where), which pointed out that Geoffrey must have been familiar with Norman French, so presumably he figured that calling a character "Merdinus" would bring hilarity rather than gravitas.