This song is discussed in Suzie Smiled... The New Wave Of British Heavy Metal. In an interview with drummer Chris Logan, author John Tucker asked if it was inspired by The Bowmen. Logan said he wasn't sure but said the lyrics were by band member John Hall, who was heavily into that sort of thing.
Apparently a Hall solo composition, the album version runs to around four and a half minutes, but Logan said the live version was much better.
So who was the Angel of Mons? Actually there was more than one. The Battle of Mons was the first major battle of the First World War, what was known as the Great War until the Second World War. Arthur Machen (1863-1947) was a well-known author - the Stephen King of his day - as well as a journalist. In September 1914 he published The Bowmen in which he recounted how the British had been assisted by phantom bowmen from the Battle of Agincourt. Unfortunately or otherwise, some people took the story literally; it was shortly republished as The Bowmen And Other Legends Of The War.
Shiva's "Angel Of Mons" was released as a single backed by "Stranger Lands."
Suggestion credit: Alexander Baron - London, England, for all above