Can anyone advise on an accordion for the left nav on the category pages? Is there a possibility to do this purely using CSS? Or do I need jquery?
Here are a few examples of Pure CSS accordion menus:
And a few examples of jQuery + CSS accordion:
While you can do more with jQuery, it's preferable not to use it if you have the luxury. In this case, assuming the mention CSS accordion menus fulfil your requirements, why should you burden your visitors and their browsers with the unnecessary script?
Note: The only instance where jQuery might be preferable is if you intend on supporting pre-IE9 browsers and some modern CSS features are not fully supported.