On my first day in my first job I had to ask my new boss if it would be all right if I left early. I explained that I was on the national executive of the Social Democratic Party, and that I held what was effectively the casting vote in a major political row.

This excruciating conversation might have been made mildly easier if the “major political row” had been comprehensible to any outsider. Instead it concerned some procedural rules governing a ballot on whether the SDP should merge with the Liberal Party.

Yet almost 30 years later it occurs to me that what we were rowing about is once again relevant. Not the details, of course, but the broad question of party splits and