I had the same problem but at a workshop at the Getaway a few years ago I was told by , was it Lisa?, that I was singing too far back in my throat.

A cultural thing evidently, peculiar to the south of England, among other places.

She suggested that I try starting each song by singing 'mmmmmmm' which is a mouth sound. When I remember to try it, it helps a lot, but I'm lazy and forgetful so, often, my sore throat reminds me to behave.