There are many companies advertising EPCs at £39 +VAT on the internet.  

That seems a good deal. Why pay more?

How can these companies provide EPCs at such a low price?

There are three typical business models here;

MODEL 1: A company employs a Domestic Energy Assessor (DEA).  The DEA will have a very low salary.  To increase his earnings, he needs to reach targets set by the company.  Once these targets are met, the DEA will be paid an extra fee.  The target levels are very high.  To try to achieve these levels the DEA needs to work fast.  
As such the DEA can only afford to spend a minimal time on the survey.  And will cut corners, not measuring loft insulation for instance.  The DEAs time on site doing the survey would be about 15 minutes.

In some cases even less.

MODEL 2: A company employs just one qualified DEA.  They employ an unqualified person to carry out the survey on your property, recording very basic data.  This "data" will be given to the one qualified DEA for him to upload.

The person carrying out your survey;

• Is not qualified (an inaccurate EPC)
• Is not licenced (an inaccurate EPC)
• Is not insured  (if he/she falls through your loft or is injured whilst at your house, you could be sued)
• Is not CRB checked (has he/she got a criminal record?)

MODEL 3: The "new boy on the Block".  Two companies are now using a system were they send out a mass email, inviting quotes, from "Energy Assessors".  It is a "bidding session".  The lowest "bid" gets the job.

These two companies do not check if the DEA is;

• CRB checked.
• Currently qualified
• Carries PII or PLI Insurance