Thank you very much. That all seems to fit well.
I have found a possible death for John Fryer (Junior) in 1868:
GRO Reference: 1868 M Quarter in ST MARTIN IN THE FIELDS Volume 01A Page 295
The age at death given is 53, which is correct if we accept the censuses that say he was born in 1815 (still haven't found his baptism), but is the place of death, St Martin in the Fields, reasonable?
I have also found a possible death for John's wife, Sarah FRYER (nee Harper) in 1872:
GRO Reference: 1872 J Quarter in STRAND Volume 01B Page 340.
It gives her age at death as 52. She was baptised in Dec 1818, so she was actually 53. If this is correct, then she was still alive in 1861 when John was apparently living with Mary Fryer. I have been unable to find a death for Mary Fryer.