Your BER assessor should have a solid backround in construction. I personally would not appoint a BER assessor who is not either an Architect, Structural Engineer, Arch. Technician, Mechanical/Electrical Engineer, Quantity or Building Surveyor, and who has less than 5 years post qualification experience in the construction industry.
It is not terribly difficult to pass the set examination, and there is too much scope in an assessment to input 'default' figures where the asssessor is either too lazy or incompetent to actually work things out. Many people who can barely read drawings, let alone have anything of use to add to the debate, have become domestic assessors.
People should employ the BER assessor at the early design stage, and ideally it would be the professional employed to design their house or extenson,or someone who can work closely with them. The software can then be used as a design tool to advise on orientation, different insulation, glazing, heating systems, renewables etc.
To get a BER cert after one has made most of the material decisions is a lost opportunity.
It is interesting to note that when setting up the certification system for commercial buildings it was decided that only persons who has a recognized qualification could apply to sit the exam and become assessors. I wonder why this restriction was not applied to the domestic market, while we were churning out 60000+ units a year?