Hi
As a matter of fact, Charles Beresford Fulke-Greville 3rd Baron Greville of Clonyn, whilst married in 1915, had an affair with his niece Cecil Violet Enid Greville, and she conceived his child.
This young lady was the daughter of Major Brooke Southwell Greville, Kings Messenger.
In 1916, it was noticed she was pregnant with Baron Greville's child, and she was moved out of the family home in Beaufort Gardens, Knightsbridge, London and along with a nanny and other servants she moved to Ankerwycke Priory, the home of her cousin Lord Guy Elliot Harcourt. She gave birth to her son Peter Charles Algernon Greville Ascroft in June 1916.
This boy was swept under the carpet, as they say, by the family so as not to cause embarrassment to Baron Greville. Aged just four years old, the family had him put into a private boys school, and his mother returned back to London, where she met and later married in 1927 Air Vice Marshall Edward Irvine Bussell CBE.
The son was later told by his Great Aunt Margaret Greville of Polesdon Lacey, who his real father was, and he assumed the name Peter Charles Algernon Ascroft Greville.
You may presume all you like, that he is not entitled to inherit the title of 5th Baron Greville, but as the 4th Baron Greville was homosexual and had no issue, then Peter Greville has every right to claim the title, as his blood is purer than the 4th Baron Greville, being Greville to Greville.
There are laws that will back this up, at the time of conception to death in various areas of residence and state legislation. At the end of the day, it was what it was, a child of the 3rd Baron Greville was born and he should be held responsible for his actions, during all years post birth.
A court of Law may recognise these facts based on evidence held by the family and their soilicitors.