ESP build problem

Hi all,
I am trying to get the heatpump monitor board to work in its intended mode, over wifi.

Following the instructions here GitHub - openenergymonitor/EmonESP: ESP8266 WIFI serial to emoncms link

I’ve downloaded everything but it won’t compile -
web_server.h:29:18: error: Hash.h: No such file or directory

Where can this be found / what else do I need to install please?