Peter Russell told William Bruffin that he liked a challenge. (HOMEVID: The Zero Imperative [+]Mark Gatiss, P.R.O.B.E. (BBV Productions, 1994).)
As Henry Van Statten admitted in an interview conducted by Mickey for his website in early 2006, the acquisition of alien artefacts was more of a challenge than his purchase of moondust from an ammo fair. (PROSE: Henry Van Statten [+]BBC webteam, Who is Doctor Who? (BBC, 2005).)