Globetrotter:
As far as I understand it, ATOL cover is only required if the sale to the customer includes the flight (charter or schedule) and other ground services (holiday package). JET2 have ATOL cover but would normally only use it for their Inclusive Tour programmes. The same as Kiss Flights, only some of their customers were covered by their ATOL, those who were IT customers. The flight only customers, even though they were on charter flights, did not have ATOL cover.
Viking are free to sell their charter flight seats to the general public or other travel organisations without ATOL cover. The CAA have other licencing procedures for airlines, I believe.