Define "secret cartel" and illustrate an example of such behaviour by
reference to a decided case showing why such behaviour is contrary to
EC competion law

Secret Cartel: A combination of independent business organizations
formed to regulate production, pricing, and marketing of goods by the
members.
dictionary.com (the proper definition)

The reason it is contrary to the EC Competition law is due to insider
trading information shared among the secret organization members. 
This gives them an advance networked connection for stock trading
information, business methods, etc.