It is always particularly satisfying when a hunch turns out to be
correct. Was there any filling in of lost parts to be done, or just
transcription and collation?

A 40-part motet by Striggio ('Ecce beatam lucem') was already known -
I sung in it in Dartington about 10 years ago.

An article here

http://www.andante.com/article/article.cfm?id=16297

asks whether the anecdote of Italian precedence and the Duke of
Norfolk is to be believed - and who knows?

The style of the Striggio motet is completely different from the
Tallis - I would say Striggio was like Monteverdi on some kind of
stimulant drug. Harmonically and contrapuntally simple, but timbrally
complex - polychoral with large instrumental complements - in other
words, modern Italian ('baroque'??) style. Whichever came first,
neither composer influenced the other in any way apart from the
multiplicity of voices. Although I suppose both used general pauses to
good effect.

~~~T~~~