Custom coded menu – weird linking issue

I’m using a completely custom theme and a coded menu which adds categories into the menu thus:

<li><a href="index.php?cat=8">Tables</a></li>
<li><a href="index.php?cat=7">Storage</a></li>

With a relating page created as ‘category-storage.php’ etc etc.

The category title is coded into each of these category pages thus:

<h1>Storage</h1>

But this weirdly only works when I link from the home page each time – if I try to link from ‘Tables’ in the menu to ‘Storage’, then the h1 tag remains ‘Tables’ etc.

it seems that – apart from the home page – whatever category link I go to first, holds the h1 tag for all subsequent category pages then linked to.

Why is this?