Forming an effective opposition to the government in case you hadn't noticed, despite that government having just a single figure majority.
And losing a few by-elections that parties in opposition a couple of years into a term usually win, and win easily.*
But hey, Labour won the London Mayoral election, thanks in no small measure to Sadiq Khan distancing himself from Corbyn.
* Actually Labour have lost just the one - Copeland. Most were 'held' but with a decrease in Labour's share of the vote.
My 1000th post. Time to celebrate.