It looks like the Flickr API has changed at some point as OWNER_ID isn't an attribute on Photosets anymore, this causes WebURL to not work for photos in a set.
EX response from the API contains
<photoset id="72157622456900037" primary="4009771355" owner="17726343@N00" ownername="chrishammond" page="1" per_page="500" perpage="500" pages="1" total="21">
As a temporary fix I've taken the latest source and replaced all instances of ownerid and owner_id from Flickr.cs and PhotoSet.CS
so far that has resolved my WebURL issues