Thanks Lisa - exactly what I needed - and I only read through it on Friday!
I have put it int the tag on the single entry page but now three things are happening that I was not expecting.
1. My nice new shiny Dirify plug-in no longer works (cleans the input field) once the related_categories_mode=“on” parameter is used,
2. The Next/Previous links, that I have used further up the page, no longer remain in the category we are supposed to be in - instead of not having a ‘next’ link on the latest entry it does have one and it is linked to the next category.
3. The guide says the that the limit= parameter will default to 100 but the related_categories_mode=“on” defaults to a limit of 10 and it seems that this the limit=”” default is overridden so you have to use a value. This one is more of an observation and as you can see in the code below was easy to remedy.
I think I have the code correct now but can someone give it the once over please.
{exp:weblog:entries related_categories_mode="on" limit="100" weblog="1" orderby="product_number" sort="desc" }
<li><a href="http://{title_permalink=/site/single_entry/}">/images/{exp:dirify input=</a></li>
{/exp:weblog:entries}