Complex sentence: Salman and his brother were known to worship at Didsbury Mosque, where their father, a security officer, was claimed to be a well-known person but a trustee of the mosque denied this and said, We don't know who he is.
Split into simpler sentences: Salman and his brother were known to worship at Didsbury Mosque, where their father, a security officer, was claimed to be a well-known person. However, a trustee of the mosque denied this and said, We don't know who he is.