They're only a bare indication of the potential heating requirement of a house and the assessors carrying out the surveys for people can have very different results, even on the exact same house. When one considers that a building contractor can be one, or have one as a worker, surely it begs the question, "Will they write up a bad assessment? give a "C" instead of something else, etc, etc, etc. Who checks them out, who surveys the survey? Very dodgy business this, but it's a must have for a seller.