We have received this answer from BRE support summarising the reasons for differences in SBEM 4 and SBEM 3.5 results:
"While the methodology for calculating the asset rating has not changed in the 2010 NCM Modelling Guide (compared to the 2006 Guide), please note that SBEM v4.1.c implements many changes introduced in the 2010 National Calculation Methodology related, for e.g., to how the auxiliary energy and lighting energy are calculated for the actual building, extensive changes to the activity database (activity set parameters, schedules, etc.), the 2010 CO2 emission factors, upgrade of ISO methodologies for calculating the heating and cooling utilisation factors and intermittency adjustments for heating and cooling demands, treatment of psi values for thermal bridges, etc. Hence, the generated absolute values of the results can differ either way from those generated by previous versions of SBEM, depending on the individual project.
Please see the 2010 NCM website (www.2010ncm.bre.co.uk) or the iSBEM User Guide (available for download from the 2010 NCM website) for the list of changes. The 2010 National Calculation Methodology is described in DCLGâ€™s 2010 NCM Modelling Guide, which is available from www.2010ncm.bre.co.uk."
A more detailed explanation is also available from: