Anybody else heard about this? I find it very hard to believe, thinking it may possibly be a pike! And Shaun Fogget is mentioned in this and several other articles, being quoted as a crocodile expert. (He too also disagrees! 
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Reading 'crocodile': Cyclist claims he saw 4ft long predator swimming in River Thames - Mirror Online
)Reading 'crocodile': Cyclist claims he saw 4ft long predator swimming in River Thames - Mirror Online