I was not thinking of Hart being in the military, but the slave trade. It was certainly still happening in the Indian ocean. In looking, I was surprised to see so many Robertson’s in India at the time.
Did Ellen remain with Thomas throughout? I cannot find a marriage between Ellen Bermingham and Thomas Robertson, but there are marriages around 1850 to other men.
Both may not have used their birth names. Travellers usually had a few aliases. If you have access to newspaper articles, you could try Thomas/John Robertson and John Hart with ‘alias’ to see if they were known by another name. Usually they would be in articles of court recordings. Some travellers married in church, especially if they married a non-traveller, but many did not marry. Travellers without close family to help, often used workhouses for births. Workhouses were expected to complete registrations, but required the mother to know specific information and cooperate. Not liking authority, many did not give the mother’s maiden name, or understand what was meant by the term.