Going back, step-by-step, (because that's how I work!) . . . . .
Utilising the new GRO Index Search, which shows Mother's Maiden Names for nearly all births:
LEFFEN, FREDERICK JOHN CAHILL
GRO Reference: 1902 D Quarter in ST OLAVE BERMONDSEY Volume 01D Page 227
Which leads to this marriage:
December qtr 1899
St Olave Bermondsey Registration District vol 1d, page 441
Leffen, Frederick
Cahill, Agnes
The Parish Record entry is available online:
25th December 1899 at St Mary Magdelene, Bermondsey after banns
Frederick Leffen, 20, Bachelor, Leather Cutter, of 28 Rothesay Street
Agnes Cahill, 20, Spinster, Tin Worker, of 40 Decima Street
Fathers: John Leffen (deceased) and John Cahill (deceased)
2 years later, in the 1901 census:
Class: RG13; Piece: 389; Folio: 35; Page: 14
17 Chloe Street, Bermondsey
Leffen, Fred Head M 20 General Labourer b Bermndsey
Leffen, Agnes Wife F 21 b Bermondsey
Leffen, Lizzie Louisa Daughter F 3 months b Bermondsey
That leads us to this birth:
LEFFEN, FREDERICK JOHN KENT
GRO Reference: 1880 J Quarter in SAINT OLAVE SOUTHWARK Volume 01D Page 281