"In a similar study published last year, 40% of 150 UK participants claimed to remember seeing closed circuit television footage of the moment of the explosion on the bus in Tavistock Square on July 7th 2005. No such footage exists." (from Bad Science)
Basically, don't trust the public on anything.
Basically, don't trust the public on anything.

Comments
Annoyingly the actual paper doesn't appear to be free online anywhere. Hooray for open research!
Footage of 'the moment' of the explosion exists - but not the explosion itself.
i.e. there's footage inside buildings where people were like "OMFGWTFWHATHAPPENEDOUTSIDE", and a lot of people have seen those - including me.