I've obviously made a really stupid error somewhere and it's knackered my admin.... the shop seems fine

I can't seem to find where it's wrong I've just got lost in folders now I'm going round in circles!!

please help! When I try to log into admin I get this -

Warning: main(includes/languages/english/header.php) [function.main]: failed to open stream: No such file or directory in /home/beautyby/public_html/shop/admin/includes/languages/english.php on line 642

Warning: main(includes/languages/english/header.php) [function.main]: failed to open stream: No such file or directory in /home/beautyby/public_html/shop/admin/includes/languages/english.php on line 642

Fatal error: main() [function.require]: Failed opening required 'includes/languages/english/header.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/beautyby/public_html/shop/admin/includes/languages/english.php on line 642
I was trying to change my date format, it all seemed to be going okay-ish apart from it was still wanting mm/dd/yyyy (I want dd/mm/yyyy and was trying to follow the thread around here.. to no avail) although it was happily prompting the customer for dd/mm/yyyy and then error messaging and asking again for dd/mm/yyyy but actually accepting mm/dd/yyyy - I've done something stupid 'aven't I!!! Help!

Here's my site if that helps - www.beautybybea.co.uk