OpenBEM has been replaced by MyHomeEnergyPlanner
See MyHomeEnergyPlanner github repository here:
OpenBEM is based on the 2012 version of SAP (Standard Assessment Procedure for UK EPC's (Energy Performance Certificate's)) developed by the Building Research Establishment for which the full specification can be downloaded from BRE here: SAP-2012_9-92.pdf
The source code for OpenBEM under GPL. You can download and install OpenBEM on your own server, you can look at and peer-review the model calculations, understand how it works, modify and improve it.