A paedophile who posed as a younger man online to have sexual chats with a child was caught by an undercover police officer.Steven Marchington, of Chadderton in Oldham, thought he was speaking to a teenage girl when he performed a sex act on himself over webchat.In fact it was an undercover officer who had joined the online forum for teenagers, posing as 13-year-old 'Holly'.Manchester Crown Court heard Marchington messaged the undercover officer under the username David UK, saying he was a