Thanks guys
Headbut - Yeah, I haven't been able to identify its type either. The silhouette of the head is hard to place against the images I've seen of Queen Vicki.
Thanks Headbut - yep all I can see is the iii too.
From comments on another forum - the wear is more likely from use as a coin rather than passage of time and water while in the ground or its closeness to the surface.