Thanks everyone, I really appreaciate it

I think you may be right Valda. After seeing Barbara F's post I found the following:
47 Roan Street, Greenwich in 1891:
John Johnson Age 42 (B S), Sweden. Head. Seaman.
Mary Johnson Age 36 Limehouse Wife.
Thomas Johnson Age 16 Limehouse Son.
John H. Johnson Age 15 Limehouse Son.
Mary A. Johnson Age 13 Limehouse Daughter.
Alfred Johnson Age 8 Limehouse Son.
William Johnson Age 6 Limehouse Son
Ada Johnson Age 4 Greenwich Daughter
Benjamin Johnson Age 4 months Greenwich Son
11 Ordnance Place, Greenwich, London in 1881:
John Johnson abt 1849 (B S), Sweden. Head. Seaman.
Mary Johnson abt 1856 Limehouse Wife.
Edward Johnson abt 1875 Poplar Son.
John Johnson abt 1876 Poplar Son.
Minnie Johnson abt 1878 Poplar Daughter.
William Johnson abt 1881 Greenwich Son.
NB Limehouse registers as Poplar which would fit with William/Theodore being the William Johnson Age 6 in 1891.
I've also been having a look at FreeBMD and I found the following:
Deaths Jun 1900
Johnson Mary Ann 45 Poplar 1c 406
I've had further conversations with my great aunt and it would appear William/Theodore had another brother named Benjamin but she is unsure of his age. She also said that her grandfather had a pig farm near Harlow in Essex so I guess everything is finally beginning to fit!