alistair It’s usually because Labour shoot themselves with internecine conflict (the later Blair-Brown years), or simply crap leadership e.g. Michael Foot, Corbyn.
Corbyn was quite interesting in a nostalgic kind of way, the political equivalent of seeing a Morris Minor on the roads in 2020, you think “oh, look at that, isn’t it interesting? I wouldn’t want one, mind” and then most of the country ended up plumping for the Alfa Romeo which is also crap and forever breaking down, but at least looked modern and promised a lot.
