User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Series 5 - the 5 Best and Worst Characters/@comment-87.52.66.152-20130109190846/@comment-5102537-20130110185010

Alice Throughton confirmed it via Twitter:

https://twitter.com/alicetroughton/status/283619155448578048

It wouldn't have made sense if it was Arthur's arm since he isn't the Lady of the Lake and certainly doesn't spend his afterlife under water.