When do main opposition parties lose seats in council elections?

My guess is a fair few journalists/pundits will get this wrong. In the interests of accuracy, therefore, here’s the list since 1983 – which is longer than I think many will report:

When local elections on their own

1992 (Labour in opposition; local elections held several weeks after general election)
1987 (Labour in opposition; local elections held several weeks before general election)
1985 (Labour in opposition)

When local and general elections on the same day

2015 (Labour in opposition)
2010 (Conservatives in opposition*)

* When voting took place.