"As CEST is a tool for guidance purposes, and not a transactional service, a GDS assessment was not appropriate,"
Only an HMRC spokesman could say that with a straight face, when they have consistently said they will stand by CEST decisions and are launching legal action based on it.
BTW a small clarification is that the decision on inside/outside is not necessarily made by the employer, the definition is that the engager makes the decision which is likely to be a recruitment agency in the majority of cases.
I see as usual for every story about contractors we get the usual assortment of AC trolls, for their benefit Contractors != tax dodgers. Yes, there are some who take the piss but that's the case in every walk of life, just ask your local MP for their expense claims for example.