The security services were indeed contacted. They attended the a/c quite quickly.
They interviewed the individual with the 'fundamentalist literature'. Here is a link to the website belonging to the authours of his literature;
http://www.hizb-ut-tahrir.org/english/english.html
The security services detained him, and interviewed the rest of the group. They stated they were satisfied to allow the others to travel, advising that the rest were not connected with this person except through business. I can't give more detail but I was happy with the explanation I got.
This whole incident took two hours to resolve and caused great distress to the other pax in spite of the best efforts made to reassure them.
While I agree with the comments being made about the extraordinary circumstances we face at the moment I still feel that the law (as it still stands) does not assist a Captain in making a pure 'judgement call' with no other firm grounds to back up refusal of carriage.
Whether get away with it will depend on how you personally handle things and on how your legal department view your actions (and whether your management choose to side with you).
I personally would recommend
a quiet word with the security people to make sure they need to interview the individuals in question properly. 
In the Terminal!
While you go on your merry way (sans the bags off course).
