LONDON (Reuters) - British police are keeping an open mind on whether more than one person was responsible for a London train bombing that injured 30 people, after they arrested an 18-year-old man earlier on Saturday.  We are still pursuing numerous lines of enquiry, and at great pace,   Senior National Co-ordinator for Counter Terrorism Policing Neil Basu told reporters.   Our priorities are ... to identify and locate any other potential suspects.   