Labour gained in an English council by-election and easily held off the Scottish National Party in a marginal ward north of the border.
It also had a healthy lead in analysis of five contests over August.
Its candidate, Sandra Peake, took a previously independent seat at Shirebrook South West, Bolsover District, Derbyshire, in the constituency of left-wing firebrand MP Dennis Skinner.
The Green Party came second.
The SNP's disappointment at Saltcoats and Stevenston, North Ayrshire, comes a week after it narrowly hung on to an Edinburgh City Council seat and indicates the party may have slipped from its high point in May's Scottish Parliament polls.
Analysis of five comparable by-elections in August suggests a projected 7% Labour lead over the Tories.
Bolsover District - Shirebrook South West: Lab 200, Green 103, C 76, BNP 43. (May 2011 - Ind 403, Lab 249, BNP 74, C 47). Lab gain from Ind. Swing 1.6% C to Lab.
North Ayrshire Council - Saltcoats and Stevenston: First count Lab 1914, SNP 1306, C 284, All Scotland Pensioners Party 211, Ind 114, Lib Dem 56, Soc Lab 43. (May 2007 - Four seats First count SNP 1629, Ind 1362, Lab 943, 902, SNP 691, Lab 683, C 554, Ind 496. Elected Lab 2, SNP 1, Ind 1). Lab hold. Swing 6.3% SNP to Lab.