Billyblue, we are all certainly descended from a common ancestor. Either Adam and Eve, an African ape, or a dinosaur-era fish, depending on your outlook. This topic is about *proveable* descent, though...
Rosinish - I do show doubt. I've had to rely on the peerage lines from about 1500 upwards. And, like any research, there's always the potential for doubt. Not sure a DNA test would work on this project, really...
chris_49 - perhaps 1/8th of your family is related to Rees-Mogg, and the other 7/8ths to his Nanny

Cell - blimey, you're taking me back. Her birth date is buried in the documents I was looking at a decade and a half ago. Fairly sure I've never seen her birth cert - the first two generations were completed by an Uncle, and I took over. I've joined everything together about them to my own satisfaction through parish records and censuses, though.
ThrelfallYorky - I have plenty of ancestors with skills who had bloody awful lives. And I can't trace them back before about 1490 - that's my earliest 'non royal' line. It's just if you are descended from royalty, you can pull things back hundreds of years further. And that's the appeal here. Not being descended from King John who, frankly, is somewhat crap.
Sloe Gin - Thanks, yes. That is my aim.