The Birth Index entry you found ...
Births Mar 1896
Sparks William Edward Plymouth 5b 226
may be this lad in the 1901 census:-
RG13/Piece: 2091 Folio: 13 Page: 17 @ 6, Parr Street
Ann Sparks 55 Widow Head of Household born England (on the Sea): on Parish Relief
Hetty Sparks 25 (Single) dau born Plymouth
Willie Sparks 5 Grandson born Plymouth
Going back 10 years to 1891, this looks like the whole family.
1891 census of Charles, Plymouth - RG12/Piece: 1730 Folio 63 Page 65 @ Parr Street
Edward Sparks 43 Head of Household born Cornwall: a General Labourer
Ann Sparks 40 Wife born Wales?
Ann Sparks 19 dau born Plymouth
Hetty Sparks 14 ditto
wouldnt be the first time I've seen a Grandfather named as Father on Marriage registration.
Looks like Edward just missed the 1901 census.
Deaths Mar 1900
Sparks Edward age 55 Plymouth 5b 208
I received the birth certificate today and the William Edward on it
is the same one (born at 6 Parr Street) as on the Census record you found Mort29

Whats more, there is no father named on the certificate, just a line drawn through the box where his name should be. Hetty was also illiterate, her name being signed by 'X The mark of Henrietta Sparks, mother'. This helps to explain the inconsistency of the way Spark(e)s is spelt.
The birth day and month are the same as one of the two possibilites that I had, though the year is different.
I'm now thinking that this probably is my grandfather, found at last I hope...