I have solved my own problem!
I already had the Custom Sidebars plugin but had overlooked that there is also a bit on the Custom Sidebard configuration page (Appearance….> Custome Sidebars) where there is a whole tab called “Default Sidebars”.
Here, you can set a Custom sidebar area (with whichever widgets you like, such as a custom menu- you need to create a custome sidebar and then alter the widgets for the custom sidebar on the widget page) and allocate it to each page/ type of page by default INCLUDING the category archive and blog pages. So, you can finally get rid of the Meta widget and archive sidebars on your category archive and blog/ news pages.
Sorry if this sounds obvious to people or is not using the right technical language! But I did come across some otehr posts with the same problem, so hopefully this helps someone!!
