"HMRC will stand by its results, if the data had been entered correctly"
Well, if it was actually possible to enter all the relevant data on CEST then maybe that would be fine. I tried it out a few months ago and found I couldn't specify exactly how I work - I'm self-employed with several unrelated clients for whom I build and administer websites. Under no circumstances would anyone consider me to be employed by any one of my clients, but CEST's rabid conclusion to the restricted amount of data it allowed me to enter was that I should be registered as employed.
The whole CEST system is a stinking pile of crap and HMRC living in cloud cuckoo land with regard to its fitness for purpose is a major source of the problem. Idiots!