Hi Claire,
At least when you get the cert you will know whom the mother is or rule this Dorothy out! Anyway here is your Frederick in 1871 (If you use Ancestry, it is transcribed as Roberson):
Bright St, Peterborough - RG101518, Folio 12, Page 15
Frederick G Robinson, Mar, 50, Railway Messenger, Lincolnshire, Gainsboro
Elizabeth " , Mar, 50, , Northams, Peterborough
Regards, Kath
You must let us all know the outcome of Dorothy's birth cert!