Dont feel guilty Sal, we have all out little (or in my case not so little!) blips, and like binny says, the fat content is ok. I bet you didn't have any side effects from it.
I can understand what your thinking in that it's not necessarily the amount of WW cake that you ate, it's the not being able to resist that you feel guilty about. This is what I also struggle with. I could really do with a session with a therapy person for this!!
Anyway, that was yesterday, how you doing today? All good!?