Hi Viv
I do not know exactly what bmd data is available through Findmypast, Ancestry has film of the GRO bmd indices. I have looked for Norman Raine in 1918 and the first quarter of 1919 and could not find him. In my family this sort of thing arises for a variety of reasons
the family do not know the exact year of birth,
the birth was registered under the first forename but the child was always known in the family by a different name,
the child was registered under the mother's maiden name because she was not married to the father at that time.
I have looked for a Norman Duke born in Sunderland in 1918 and not found him. I have also scanned the 1918 films rather quickly looking for a male Raine with a mother called Duke and could not find one. (It might be worth trying this again more carefully and slowly than I did.)
Freebmd has a Norman Raine born in Sunderland but the date is very different from the family date
Births Mar Q 1905 Raine Norman Sunderland 10a 725.
This predates the inclusion of the mother's maiden name on the index so there is no inexpensive way to tell if she is or is not a Duke. However Freebmd also has a marriage between Lilian Atkinson Duke and John Albert Raine in Sunderland in the same quarter - reference Sunderland 10a 827.
There is the slight possibility that Norman has been missed off the national index as a result of a copying error. Fortunately you can search the Sunderland Registrar's index on
http://www.sunderland.gov.uk/pages/registrars/There is no Norman Raine listed as born in 1918 but there are two other male births, Charles Raine and George Raine. Charles is listed in the national index as Charles E, mother's name Cowan, so he is not your target. George is missing from the index and unfortunately the Sunderland search does not reveal his mother's name. So it is just possible that George is your Norman but to check you will need to negotiate with the Sunderland Registrar. If they are very kind they may be willing to tell you over the phone the name of George's mother.
Good luck
Gobbo