You can just enable settings->data log in cgwatcher and look there after lease, or in miner.log in cgwatcher folder, there's some info too.
+You can import that data log to excel to make graph from it, and you see immediatelly if there was downtime.
Not much convenient, but good enought for now.
Not using Windows, nor will I. I like running Xubuntu from a small usb stick + I want SSH.
+ Reading a log after the lease is pointless. I want to check it during the lease when I encounter a problem so that I would be able to give extended minutes in return for the downtime.
Once the lease is over, it would be too late.