Cornwall is going to be one of the counties to watch at the general election.

All of its six constituencies are marginals – none more so than Camborne & Redruth, where the majority of Tory MP George Eustice is a miniscule 66.

It was in Eustice’s constituency that voters went to the polls on Thursday to take part in a council by-election. The contest was for a seat in the Illogan ward.

It was the only one of this week’s by-election seats to change hands. And intriguingly, it was won by the Liberal Democrats, who went from fourth to first to snatch the seat from the Tories:

Not only did the Tories lose the seat, they went from first to third, coming behind the Lib Dems and Mebyon Kernow (the Cornish independence party) who even didn’t stand when the seat was last contested in May 2013. The result saw a sizable swing of 10.2% from the Tories to the Lib Dems.

It makes no difference to the balance of power on the mammoth 123-seat Cornwall council, which is run by a coalition of Liberal Democrats and independents.

But I imagine it might make a difference to morale within the some of the local Liberal Democrat parties, for whom Cornwall could prove to be a particularly bloody battleground next May.

The Lib Dems currently have three MPs in the county, and they are all vulnerable to the Tories: Steve Gilbert in St Austell & Newquay (majority 1,312), Andrew George in St Ives (majority 1,719) and Dan Rogerson in North Cornwall (majority 2,981).

Recent opinion polls carried out by Lord Ashcroft suggested the Tories are currently in front in both St Austell & Newquay and St Ives.

There were five other by-elections this week, and in every case the sitting party retained the seat.

Labour held on in a closely-fought ward of Barnsley council, achieving a tiny 0.5% swing from the Tories:

Labour has a 43-seat majority on Barnsley council.

The party also held two seats on Tory-run Cheshire West and Chester council in the wards of Boughton and Winnington & Castle. In both cases there was a small swing away from Labour to the Tories, and Ukip came in a distant third place.

The Conservatives held the other two seats up for grabs. One was on North Hertfordshire council:

There was a 2.5% swing from Labour to the Tories, who have a 19-seat majority on the council.

The other seat was on Torridge council:

Here there was a 5.5% swing from Conservative to Labour. Torridge is run by a minority Tory administration.

Taking all the results from this week’s by-elections, an analysis by the Press Association of six comparable results suggests a projected nationwide Labour lead of 4.6%.