Can we have a "progressive alliance" in Westminster?

Nicola Sturgeon wants the SNP, Labour, the Lib Dems and the Greens to unite in order to get rid of the Tories. It's a nice idea but Labour and the Lib Dems are stubbornly against it and probably hope they can form their own formal coalition. This will help Nicola anyway as she can say even the slightly better English parties don't want us to have a say.