Many thanks for your responses.
Viktoria, I love the story about Handsome Ned, and it's certainly possible that Robert may not have held his second wife in high regard. I've just read up on the Married Women's Property Act passed in 1870, which confirms that the act wasn't retroactive, so any property (including money) that Mary had when she married Robert in 1852 was still his to dispose of as he so wished in 1877 when he wrote his will.
Robert's first wife was Selina Brown, her father William was a shoemaker who died in 1841, (before Selina had had any children), and I doubt very much that there was any money to inherit from that side of the family.
Thank you aghadowey for your suggestions. Mary's children were born between 1828 and 1837, so the youngest would have been 40 when Robert made his will in 1877. I'm struggling to find out what happened to them all, but I do know that one of them emigrated to Canada in 1856. Mary's first husband was Thomas Morley (aka Virgo), who died in Brighton in 1837. He was the eldest son of Thomas Morley and Lydia (nee Virgo), and as they were still both alive when their son died, it is quite possible that they contributed to the upkeep of his widow and the five children. I haven't as yet found a will for either Thomas or his father (also Thomas), but I shall certainly explore that possibility.
Mary's father was Thomas Tugnett, shown as a labourer on the 1841 census living in Wiston Sussex, and died in 1845, and I doubt very much that Mary inherited any money from that branch.