Hi Sam,
I don't find it the easiest site to move through. I just put window tax in the search box - the first item was about window envelopes and the next about tax credits!
However, just above that bit is a section that says 'narrow your search results by subject'. If you pick Tax, Money and Finance you'll start to see things about window tax and you can narrow the search yet again if necessary.
Down the right hand side of the page there are often links to study guides which could be useful.
In addition you could go the the Access to Archives section of the site which will lead you to documents held at local records offices so you can see what's available in your counties of interest.
Christine