Unlike opinion polls, council by-elections show us what people who actually show up to vote are thinking.
There have been 168 council by-elections since the Local Elections in May 2023. Here are the aggregate council wards won and lost:
LDM: 52 (+19)
LAB: 50 (-2)
CON: 29 (-18)
GRN: 16 (+6)
IND: 16 (+3)
LOC: 5 (-2)
PLC: 2 (-1)
SNP: 0 (-5)
(LOC = local candidate/local party. They're different from IND who are independant candidates who don't belong to any political party).
One caveat - because by-elections are triggered by the resignation or death of a sitting councillor, they're not spread evenly throughout the country, so might not be representative of the country as a whole. That said, 168 is a good number of wards. Each ward will have an electorate of about 3,000 voters. So these 168 wards represent about 504,000 voters.
The first thing to leap out is that despite their vast opinion poll lead, Labour haven't made any net gains.
Instead the LibDems have been surging, at the expense of the Conservatives. The Greens are also doing well - they are the go-to party for voters unhappy with Labour and the SNP.
The big loser, apart from the Conservatives, is the SNP.
So, based on this, Conservatives will lose their majority, mainly due to seats lost to the LibDems. The SNP will lose to Greens and Labour. Labour might lose seats in England to the Greens. It looks like hung parliament territory to me.