He might not quite say as much, but I suspect that Mark Thomas probably got a perverse kick out of discovering (via a subject access request under the Data Protection Act) that he features on the police's database of so-called "domestic extremists". Nevertheless, the extent to which his activities have been monitored was no doubt alarming - especially as some of the information recorded was untrue. If attending peaceful protests and signing a few petitions makes you a potentially dangerous subversive, then my name should be on the list too.
On the subject of covert monitoring, here's a fascinating article revealing how the FBI doggedly investigated Jean-Paul Sartre and Albert Camus to find out whether their existentialist musings were actually a cover for Communist activities.
(Thanks to Adam for the links.)