Just to clear a little detail up, the CAA do sometimes telephone first. They called me once re a Stansted airspace infringement. As it turned out a mate was flying my Yak that day; I gave them his phone number and they just had a little chat, NFA.
I think that perhaps they follow these things up because they have to, as an earlier poster mentioned, and a phone call is their informal way of seeing what substance there is to it. Sort of fits what you said, M14P.