At least one of them, Mark Stone (who was paid £100,000 a year by police), spent seven years as a leader of the organization he was informing on.
This of course, can and does happen in the States. Consider the multitude of supposed revolutionary Marxist parties here in the US. Now that a real life capitalist crisis is here they’ve apparently dropped off the planet and are nowhere to be seen. How curious. Or maybe not so curious at all. I wouldn’t be surprised at all if the leadership of many such groups has been compromised for years.
Here’s a tip for those in such groups. Beware of the a) angry extremist who keeps urging for increasingly radical action, especially if it crosses the line into illegality and b) the ideologue who paints everything in rigid doctrinaire terms thereby ensuring the party will remain tiny and ineffectual. Also, check to see if the leadership of the group appears to living beyond their means or if the funding of the group is murky and undisclosed.