Custom menu suddenly not supported (piano-black)

Hi,
I have been running WordPress with a theme called ‘piano-black’ for about a year now. When I set the site up all that time ago, the theme supported custom menus, and indeed I set up a bunch of them and it all worked fine. Even a month or so ago I went in and updated my menus slightly and it was all still good.

However after the most recent update I don’t seem to be able to change them at all, instead getting the following message: “The current theme does not natively support menus, but you can use the “Custom Menu” widget to add any menus you create here to the theme’s sidebar.”

Now this is confusing the hell out of me – I’m pretty sure up until this version they were still working fine. And even though I am getting this message now, the custom menus are still displaying on my blog 100% correctly. I just can’t change them anymore.

I know this is going to be a bit hard to troubleshoot due to being a custom theme – I can provide any details required in order to help figure it out.. any help or ideas would be greatly appreciated.