I've no idea whether similar research has been carried out at British newspapers, but a US survey of traffic on newspaper websites comes up with the following figures. Just 37% of newspaper site users are regular visitors, and only 27% visit daily.
The research, carried out by Belden Associates, also found that audiences have aged five years over the past five years, which suggests that the websites are drawing people who are already newspaper readers. There's more to be found on this survey at Media Life Magazine.
But the figures on returning, and non-returning, visitors are a surprise, at least they are to me. I've got a hunch that the figures here for national newspaper websites would show greater regularity of usage. Anyone know? Anyone want to find out?