When I enter this url https://www.themesandco.com/wp-content/themes/customizr/inc/css/fonts/fonts/genericons-regular-webfont.ttf in the themesandco (Customizr) site, it asks me if I want to open the font in Font Book (I’m on a Mac).
When I try to open yours with https://martialartsplymouth.co.uk/wp-content/themes/customizr/inc/css/fonts/fonts/genericons-regular-webfont.ttf it asks me what I want to do with the binary file.
So your Genericons font file is not being recognized as a font.
Your files may be corrupted (though the sizes of the files are the same). Or maybe it’s a permissions issue on your folders.
I didn’t download them, but you might wish to: to compare them and see what’s different about yours.
A quick fix might be to reinstall customizr through the admin panel (activate twentytwelve, delete Customizr, reinstall Customizr via the dashboard, and re-activate it).