Just another update on my progress - after hitting a brick wall I decided to go back to the original wedding certificate and look at the witnesses to see if they could give me any clues.
The two witnesses were Alfred Carey and Hilda Shephard. I've managed to find an interesting link from Alfred Carey that could help find a bit more about William Ryan.
I've found an Alfred Charles Carey who was born in 1881 in Rotherhithe to Elizabeth and George. He is listed in the 1881 census as well but in 1891, Alfred disappears from the family. I was able to find an Alfred Bartholomew Carey who lists his birth as 1879 and is at St Mary's Orphanage in North Hyde, Heston, Middlesex. Coincidentally, there is also a William Ryan who is listed at the same orphanage in the same census who is the right age my g-grandfather would have been.
In the 1901 census the only Alfred Carey in London has married a Rose Carey and is working as a wine porter. He can also be found in the 1911 census living in Lambeth with Rose and a daughter.
It's a bit of a long shot but is there a possibility that the Alfred Carey on the marriage certificate is connected to William through this orphanage? And if so, any tips on how to find some records to prove/disprove this theory?