Remote Water Temperature Monitoring

[Trevelyan]

I use a Hive smart thermostat to remotely control the heating and hot water in a holiday rental property. It works well, allowing us to turn things off when the house is empty, but get the house warm and hot water cylinder heated up when guests are due. The one thing missing from it though is an indication of the actual hot water temperature. The guests currently in the house have just called to say they've got no hot water, and although I can tweak things in the app I've got no way of telling whether the hot water is now starting to heat up, or even if the boiler is even running.

Does anyone know of a basic temperature sensor I could strap to the hot water cylinder with possibly a second one on the boiler which I could use to see the temperatures via a phone app or over the internet?