@GraemeD,
British citizenship by descent
"British citizenship by descent" is the category for the children born outside the UK to a British citizen. Rules for acquiring British citizenship by descent depend on when the person was born.
From 1983
A child born outside the UK on or after 1 January 1983 automatically acquires British citizenship by descent if either parent is a British citizen other than by descent at the time of the birth.
Before 1983, as a general rule "Citizenship of the UK and Colonies" (CUKC) was transmitted automatically only for one generation, with registration in infancy possible for subsequent generations. Transmission was from the father only, and only if the parents were married. (See History of British nationality law.)
https://en.wikipedia.org/wiki/British_nationality_law