After a quiet couple of months, the polling averages for all main parties are on the move – and it’s Ukip alone that has reason to be cheerful.

The party has put on 1.7 percentage points since August, increasing its average from 13.2% to 14.9%.

This is the second highest average Ukip has ever achieved; its highest remains the bounce it enjoyed in May 2013 off the back of the local elections, when the party hit 15.9%.

The other three parties have all seen their rating drop.

The Tories have suffered the most. Their average has fallen 1.4 points from 33.2% to 31.8%. The party hasn’t rated this low since July 2013.

Labour dropped 0.8 points from 36.4% to 35.6%. This is the party’s second-lowest monthly average since 2010; its lowest remains 34.9%, recorded in May of this year.

For the Liberal Democrats, September brought the latest in a sequence of all-time lows. Their average of 7.7% is 0.2 points below their previous record low of 7.9% in June. 12 months ago the party was at 10.2%: a score that now feels positively dazzling.

If mirrored at the general election next May, September’s poll averages would produce a Labour government with a majority of around 44. If there was a uniform swing, the Lib Dems might be reduced to as few as 20 MPs.

A uniform swing would also mean no MPs for Ukip. The result of next week’s Clacton by-election will probably suggest otherwise.

By plotting the averages from the past 18 months, the acceleration in the Lib Dems’ decline is rendered even more stark. You can also see how the gap between Labour and the Tories has first narrowed then stabilised.