I’m going to need to remember this, as it runs slightly counter-intuitive to what I was expecting (although makes sense).
Code compiled in App_Code is available globally, so any other piece of code can reference it.
Code compiled in code-behind (i.e. the .cs files ‘behind’ ASPX pages) is only available to its corresponding ASPX page unless you explicitly reference it.
This can be done by adding the following to the ASPX page:
<%@ Reference Page="......link to .aspx" %>
One to remember.