Help Needed To Troubleshoot a Strange WP Issue

Hey guys,

So I'm after your help to troubleshoot this issue that one of my customers is finding they're having regularly.

I've got a property website designed and installed over at letmelettings.co.uk and the front end works perfectly well. Loads pretty quickly, not much going wrong there. When I log in to the back end from my office, on my awfully slow internet connection, the admin sections also work fine!

Once I visit their office, using their great internet connection, the front end is still fine, but the whole admin area becomes completely unusable. It won't load a thing in the back end. Obviously this is a problem, they can't add any properties to their site (and I'm about to lose some business as a result, me thinks)!

They're currently running Windows 7 (64bit), have tried to access the site's admin area using every major browser and yes, they've tried turning it off and on again.

Do you guys and girls have any suggestions? We've either tried or contemplated everything we know between two of us in the office here and have come up with nothing useful!

Thanks!! =)

Scott.