There is a self sustaining population of introduced ruddy shelduck in the Netherlands - and these birds show regular migratory movements.
Its likely that some, if not most, (or even all?) of our ruddy shelducks are from this stock. I agree that it's likely that there are genuine vagrant ones, but which ones are they?
Another likely candidate for addition to C5 is bar-headed goose. Spring peaks in arrivals (sometimes of large groups) in eastern Scotland and the Northern isles - and much less chance that these are genuine vagrants...!