When was the first child born? Were all the children registered with their father's surname? Have you looked for marriages to other people - they may have been living together because one of them, or both, were already married to other partners.
Not all marriages got recorded in the GRO indexes. You might want to ask if the local register office could help, or see if the relevant local family history society has an index of marriages - several do and offer an inexpensive search facility.
I do sympathise. My gt grandmother remarried after my gt grandfather died. He died May 1879 and she appears in 1881 census with new husband, giving birth to their first child together a few months later. New husband had rare surname and I can't find a previous marriage for him, but can't find his marriage to my gt gran either. I've searched many parishes in Holloway, London, where she was in 1879, and Greenwich, which is where they were in 1881 to no avail. The relevant local register offices don't do searches for genealogy purposes. So I'm stuck.
