Problem downloading OS X 10.7.5

I'm using a Macbook 2008.

I've recently had my Mac's Ram and OS upgraded to Os X 10.7.4. My Software Update is telling me that I should upgrade to OS X 10.7.5.

However, when I try and download and install the new OS I get error message. "The Update could not be verified. It may have been corrupted or tampered with during downloading. The Update will be checked again next time..." But this happens every time.

Can anybody offer any advice?

Thanks