I got the following error when I upgraded WC to the latest version:
Fatal error: Class 'WC_Payment_Gateway' not found
This results in the whole site going down. I think the problem is, when you upgrade WC, the WC plugin is deactivated for a short while, then the SagePay plugin can't find the class, and you get an error. I guess a way around this is to deactivate the SagePay plugin before you update WC, then reactivate. But this isn't easy to remember.
I think the SagePay plugin needs to check whether 'WC_Payment_Gateway' is a valid class to prevent this.