Hi Peter,
A couple of weeks ago there was a report of a family pary of Brent Geese, 2 adults and 3 juvs from memory, on Elton Res.
The same day I had a juv Brent Goose with Canada Geese up on Ashworth Moor Res and my thinking was that this lone juv had probably got split up from the family party due to the poor weather of the day.
I suspect the Brent Goose you had on Cowm is this lone juv and hopefully a genuine bird. 
Regards,
David. 

Hi chaps

The brent goose was still present this morning.  I have posted some photos here http://www.flickr.com/photos/26821757@N02/2989686110/in/set-72157608541237569/ showing that the bird is unringed.  It does seem rather tame though and it always stays around the left hand corner of the res as you look from the dam wall. Also at Cowm today were the following

mallard - 27
tufted duck - 1m
goosander - 1m
grey partridge - covey of 5
black headed gull - 60
common gull - 2
green woodpecker - 1
meadow pipit - 2
grey wagtail - 1
redwing - flock of 50 over
fieldfare - flock of 16 over
starling - flocks of 5, 40, 40, 100 & 12 over
goldcrest - 3
jay - 2

cheers Kevin