Liquid malt extract? Or dry? Brand? I don't recall if Munton's dry has a low fermentability. I think it does, but I'm not positive.
I don't use White Labs yeast (foolishly brand loyal, not for any other reason) but that yeast may be a low attenuator. Many of the English strains are.
Did you make a starter? Or just pitch the vial?
If you're underpitched with a low attenuating yeast, you may be done. If the yeast has dropped, I don't think you'll get it to go any further.
I've used the Wyeast ESB yeast, which I believe is supposed to be the same strain, and once it flocculates out it is hard to get it back into suspension to keep working. And it floccs like crazy.