Cannot access App Store after upgrading to Mountain Lion

Hi all - hope someone can help. I have a new Macbook Air and upgraded recently to Mountain Lion. Since then I cannot access the App Store. Error message is

'Cannot verify a secure connection to App Store, continue? ' If I continue a series of flashing pages occurs. I have tried changing the date as some responses suggest but nothing seems to work. I would be so grateful to fix this as I am keen to download final cut pro x to my new machine.

All help gratefully received