That's a good idea getting the cert for the address - possibly also William's birth cert on 18 Dec 1879.
My guess is that they were left off the original FamilySearch index, but that they are probably in the census. Do you live near any of the libraries which have the census films or fiche?
You could search Douglas and Ramsey the way we used to do it in the old days - page by page, but an address would certainly help. There used to be street indexes to larger towns.
You could also search scan by scan in Ancestry, but I've found a whole district missing there, so the PRO fiche would be better, or the ones commercially available on CD.

Let us know the address when you get it?

Sue