Hi Leanne,
This one is on 
http://rootsweb.freebmd.com:
Births Sep 1879 
FISHER  Sydney Everest    Wandsworth  1d 725 
In 1879 the registration district for London & Surrey was 1d, so I would think he is a likely one to check if you can afford the cost of the Certificate. The 1d 725 reference numbers are what you will need to get the Certificate.
I assume you have seen where the family was in 1881? It certainly confirms Sidney as being born 1879:
 John FISHER   Head   M   Male   32   Bury St Edmonds, Suffolk, England   Carman     
 Harriet R. FISHER   Wife   M   Female   35   Marylebone, Middlesex, England        
 Alice FISHER   Daur      Female   7   Sutton, Surrey, England   Scholar     
 George FISHER   Son      Male   5   Sutton, Surrey, England   Scholar     
 Sidney FISHER   Son      Male   2   Sutton, Surrey, England        
 Henry FISHER   Brother   U   Male   22   Bury, Suffolk, England   Carman     
~~~~~~~~
Source Information:
  Dwelling   12 William Road 
  Census Place Sutton, Surrey, England 
  Family History Library Film   1341178 
  Public Records Office Reference   RG11 
  Piece / Folio   0758 / 80 
  Page Number   41 
Hope it's helped.
OzzieJoan