Peter Maxwell Davies, wasn't it?
Max does indeed get the composer credit in the film titles.
However, AFAIK much of the music was concocted in collaboration with David Munrow, whose ensemble (the Early Music Consort of London) play on much of the soundtrack. Munrow in turn looked to various Renaissance, early Baroque and medieval composers (including a lot of Praetorius, with which the film opens) for his dots. And of course Praetorius was a publisher as well as a composer - the dance suites in his
Terpsichore are clearly French imports (the different ballets have French titles), although the actual authorship of these pieces is probably now untraceable for the most part. Praetorius certainly deserves an arranger credit, at least :) [Praetorius never bothered much with crediting the composers he included in his dance collections - even though several of the pieces - such as Schein's LA BATTALIA - are clearly not Praetorius's work, and many of them date from up to a century beforehand). There's also quite a bit of Munrow's own composition and improvisation on the soundtrack - there's a longish bit of improvisation on a Chinese
sheng, for example. There are also a few medieval 'hits' on the soundtrack, which are known from different sources - such as "Schiazula Marazula", a pseudo-African (?) piece which was published in all kinds of dance albums from the 1550s onwards - Praetorius also includes it, but in a very watered-down contrapuntal arrangement of his own. It's not impossible that the tune (and it's only a melody-line - presumably played over a drone accompaniment?) had actually come from the Maghreb - shawm bands (shawms on the top lines, drone trumpets below, and a load of drums) used to busk their way round Europe at the time, bringing their own repertoire with them... they have survived to this day in parts of Catalunya.
[Way back when, I used to have lessons with Alan Lumsden, one of the core Munrow players - who told me who had played and composed what on The Devils. PMD was a friend of Munrow's, and it was all done in friendly collaboration, I believe?]