To use, download the OS Terrain 50 data from Ordnance survey (https://www.ordnancesurvey.co.uk/opendatadownload/products.html)
Unzip this is src/main/data so you end up with a directory structure like:
src main data terr50_cesh_gb data hp hp40_OST50CONT_20170713.zip hp50_OST50CONT_20170713.zip hp51_OST50CONT_20170713.zip etc ht hu etc