New theme – functions.php not included in single.php

Hi,
I’m developing a new theme from scratch and have set up a theme directory with:
style.css
index.php
home.php
single.php
single-<my-custom-posttype>.php
header.php
footer.php

The setting for the WP installation are pretty much vanilla at present.

The problem I’m having is that when I define a function in functions.php it is available in index.php & home.php but NOT in the single templates.

So, for example, I have a template tag defined in functions.php as follows but cannot make use of it in single.php

if (! function_exists('my_function')) {
  function my_function() {
    echo "Something";
  }
}

I have followed the documentation on setting up a theme and have ensured that wp_head() is called before the closing head tag.

Naturally I expect WP to ‘just work’, that being one of the major attractions, so I’m a bit flummoxed here!

Any suggestions?

Thanks,
Ian