Thanks for the heads-up on the Gallery field Jonathan, but my website’s gallery is quite complex, requiring a few checkbox custom fields to control things like cropping for each image. Hence why I’m using the Repeater plugin.
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.
I should have said – I’m up to date with my versions, WP v4.1.1, ACF v4.4.1