Thanks Kita. She is mainly terrier. Mum was a Staffordshire bull Terrier, and her dad was a Rottweiler / lurcher cross.
I gave her a bath tonight as I was thinking perhaps it's just muck. We walk a lot in the country and she rolls in anything she sees. I don't usually bath her as she doesn;t like it, so had a theory it could be general dirt.
As I said it does not seem to be causing her any trouble, but wondered if it may break out into something more serious, or be an infestation.