This helped me realise that ACF was built into my template theme: http://support.advancedcustomfields.com/forums/topic/blank-page-after-activation/
to solve this, I changed my (theme)/admin/acf/acf.php from
define( 'ACF_LITE', true );
to
define( 'ACF_LITE', false );
Now I can see the ‘Custom Fields’ menu in WordPress without having to activate the plugin.