I appears to be a cock-up on the part of of the British security services - they couldn't charge the suspects without charging the spy, thus blowing his cover, of course his cover is blown now!!
Makes me sick to see a terrorist group gloating.
Related story:
http://www.freerepublic.com/focus/f-news/1542380/posts
How they kept informants from the IRA and INLA in the same wing as ones from Protestant paramilitary groups, like the UDA.
It sounded like an odd law enforcement strategy, to be honest.
Plus, according to him a lot of their testimony had been thrown out in the ensuing years.