Chris - I copied the information below from the website at acoustic guitar magazine -

The only steel-string acoustic guitar Nick Drake ever recorded with was a small-bodied Guild M-20. It is pictured on the cover of Bryter Layter. The M stands for mahogany (back and sides), and the top was burgundy-stained spruce. The advantage of smaller-bodied acoustics (as opposed to dreadnoughts) is that they're generally easier to record, with a nice balance across the tonal spectrum. Drake's guitar never required compression in the recording studio. He also recorded several pieces with a nylon-string guitar, but its identity is unknown. Most likely it was a borrowed guitar. He accompanied himself on piano on one recorded song: "Saturday Sun" (Five Leaves Left).

All of Drake's sessions were recorded at an eight-track studio in London called Sound Techniques and engineered by John Wood. Five Leaves Left and Bryter Layter were produced by Joe Boyd, Pink Moon was produced by John Wood, and Time of No Reply was produced (after Drake's death) by Frank Kornelessun and Joe Boyd. In a BBC radio special broadcast in June 1998, "Fruit Tree: The Nick Drake Story," Wood commented that after Drake's death, scores of acoustic guitarists asked him to engineer their recording sessions in the hopes of duplicating Drake's sound. Each of them would leave the session frustrated. As Wood said, the best mic in the world is not going to make you sound a better guitarist than you are. Drake had all the talent and ability, and reproducing his "sound" was quite easy. It all came from Drake.