Dear All, Dowland (1610) says use a 4th course string [probably less than .80mm] for the first two frets, a 3rd course string [perhaps c.62mm] for the third and fourth frets, a 2nd course string [perhaps c.50mm] for the fifth and sixth frets, and a first course string [perhaps c.40mm, he used a double top string] for the rest (i.e. the 7th to 10th frets, as he was talking about a 9c lute with ten tied frets on the neck). All diameters are just rough estimates, based on my experience with modern gut strings/tensions.
I have little doubt he was using double frets, which are clearly shown in all kinds of iconography. The use of very thick single frets seems to be a modern invention. Thomas Mace (1675) mentions single frets as a new invention, perhaps his own - but there seems to be little evidence that it caught on until the 20th century. Peter Nightingale's buzzing problems could be due to several factors, obviously fret wear, as has been mentioned. Another factor which has not been mentioned so far is that if a fret is loose, it tends not to sit tight to the fingerboard near the edges, and is therefore higher than it was when it was tight, causing buzzing when the string is fingered on the next fret down (towards the nut). Best wishes, Martin
