Female first? Not if you were a member of the royal family. When it came to inheriting the crown, ‘male-preference primogeniture’ applied, which meant that younger sons always took precedence over older daughters. This remained the case until just a decade or so ago.

Maggie Ballinger

Maggie Ballinger

It explains why, in Britain, there have been only seven queens in their own right since 1066. This compares to more than sixty kings of England and Scotland − a far from even balance.

In fact, for centuries, the idea of queens as rulers was complete anathema, which explains Henry VIII’s desperate desire to produce a son.

Things might have played out differently, had gender discrimination not existed. The eighth Henry himself had an older sister. So, too, did Charles I (who literally lost his head) and George III (who periodically lost his mind). Queen Victoria’s eldest child was a girl, Princess Vicky. Vicky’s son was Kaiser Wilhelm II of Germany. Had his mother become Queen of England and he had subsequently become our king, the First World War would never have happened . . .

Females who were first in the line of succession to the throne were never guaranteed to become queen. This was the case with our present monarch. Up to the time of her father’s premature death in 1952, Princess Elizabeth was always ‘heir presumptive’ and never ‘heir apparent’. When, over evening drinks, my husband happened to ask about the difference between these two terms, it raised an intriguing premise for a novel.

What if Princess Elizabeth had had a much younger brother? Although rare, later-in-life pregnancies (when nature sometimes has a final fling) are certainly possible. Halle Berry, for example, was 47 when she gave birth to her ‘big-surprise’ son Maceo.

Enter baby Prince James. The subject of much wine-fuelled speculation, he dominated our conversations. Born three months after the arrival of Prince Charles, ‘Uncle Jimmy’ had a slightly older nephew. How would the boys’ relationship have developed? Would – indeed, could – our fictional British sovereign have changed the course of history? And, if the child king were eventually to marry and produce issue, did we need to invent a plot twist that would restore the monarchy to its present-day form? (Spoiler withheld.)

Loads of research was required. Reference books, the internet, academics and friends ‘old enough to remember’ were consulted. This wasn’t just about historical events, or complex matters of constitutional law. What terms, in those days, had yet to reach the dictionary? In the 60s, informal musical performances weren’t referred to as ‘gigs’. Was ’role model’ in common usage in the late 1940s? All the findings, too good to waste, are included in the book’s well-received set of explanatory notes.

From devising a name for a Scottish journalist − we wanted an anagram of Martin Bashir and came up with ‘Rab Rainsmith’ – to factoring-in subtle hints about the eventual shocking denouement, the whole process was enormous fun. This, hopefully, is reflected in the finished tale. Writing the sequel is proving even more entertaining.