I don't think I'll ever be able to prove or disprove my family myth, as this is how it goes :
A lass of unknown name was either secretly married to, or was the mistress of, a Lord, possibly of Raby Castle Vane family. He fell off his horse, died, and she was done out of her inheritance.There were no offspring of the alledged union.
Well, one of the Lords Barnard did die after a riding accident,
but also the Tempest family of Wynyard, lost their only son
John Wharton Tempest (1772–1793) (the subject of a painting by George Romney), as a result of a riding accident. I am favouring this second one as perhaps the more likely, as the story seems to have originated with the Richardsons from the Rainton /Lamesley area.
Although a coal miner's daughter meeting a Lord is a bit unlikely
There's also Crosier Surtees as an outside chance. Lieutenant Crosier Surtees (1740-1803) inherited Redworth Hall near Heighington DUR. Crosier was a drunkard and womaniser, according to Wikipedia! In about 1800 his wife Jane left him. He moved into the house of his mistress in Pennington Rake and had several more children. Suddenly in 1803 he died when he was returning on his horse, in a drunken state from a banquet with Lord Barnard in Raby Castle. On the moors he tumbled from the saddle, fell into the water, and froze to death.
Anyway, there are just too many aristocrats with Raby connections falling off their horses to choose from!