If you stand by the ‘Biffa’ bins by the entrance roundabout to ‘scope the Ring-necked Duck in the NE corner it could be up to 850 metres away from you. (I’ve got access to an Ordnance Survey website for schools which has a really useful measuring tool on it.) At that distance the contrast between the white on the fore flanks compared with light grey on the rest of the flanks is probably the best feature to look for.
Pete C