Surname of British comic actor (Chris Langham) convicted of downloading child pornography hence
To do a Langham - to download kiddie fiddler flicks

Also charged with, but not convicted of, attemtped buggery on a minor (alledged to have attempted to take an underage female up the wrong 'un in a hotel shower)
Hence definition two:

To do a Langham - to attempt to take a girl up the wrong 'un