Thanks Gadget,
Yes, I broke my own rule by listening to information passed on by another family member without double-checking it against the available records. This is why I don't post family trees on Ancestry etc and I never take for granted anything I see posted there unless there are records that verify the provenance !
Anyway, even though Robert Jnr was apparently born in Q1 1854, I think that marriage record is the most likely one and it's worth applying for the certificate which will hopefully confirm the residences of both parties.
Thanks again, Gordon.