Not a member of the Sparrow family, but from the more exotic Accentor family.
That's wierd, as two different bird books I own say they're hedge sparrows :shrug:
Andy!! Where you bin? :shrug:
It stems from ancient times when practically any small brown bird was called a Sparrow, Dunnocks got stuck with this tag.
If you look at a Dunnock compared to a genuine Sparrow, like a House Sparrow, you can see they are not related... Dunnocks are primarily insect eaters with soft pointed bills, where-as genuine Sparrows have strong conical bills for crushing seed. Dunnocks are not flock birds like Sparrows and lead more solitary lives.
cheers
Andy