Having looked at a few solutions for monitoring my web applications I have settled on the following cross platform solution that comes with a useful phone app (Android and iOS). The free account will monitor up to 50 servers, web apps, sites etc. Posted here as it may be of use to others. You can buy a paid for version, and also buy SMS credits if you would rather have text messages instead of email notifications, more servers and more frequent monitoring but 50 servers is more than enough for me.
I use Server Ranger (love this app, and it’s by our own @Gavin Smith ) to have a HUD on a different monitor at my desk. I use JetPack for WordPress sites where I make use of other JetPack features. I use UptimeRobot for monitoring the rest.