A family story of our descent from a well-known local squire

As a child my Mum told me that her grandmother had told her that her grandmother was the daughter of a local squire. My Gran further told me that this girl had run off with the coachman and the upper class family wanted no further contact. The couple had eloped and went ‘over the hill’ to seek sanctuary with the former chaplain of the parish.
So I started expecting to unearth a story with a spoilt rich girl struggling to cope as a coachman’s wife.
BUT on one tack as a beginner I soon found that all legitimate daughters of the local gent were fully accounted for. And I followed my known lines back too, and got to the right village with full documentation not leaps of faith, but that was slower.
So obviously the girl was not a legitimate daughter which leaves me with the question (and brick wall) of who was her mother.
I have a fair bit of confirmation through DNA of the local gent, who was of royal descent (from both Plantagenet and Tudor royal lines) which is not rare since I understand that about 25% of the country are. However despite this royal connection my Mum says I must not tell everyone about the illegitimacy so no surnames or places in this story. Most family trees have some illegitimacy but she was brought up in an era when it was too shameful to discuss. Whereas in 1811 it was better to have a richer man as a father than marriage to a pauper.