I've found it!!
There is a colour picture on this page:
http://www.romanarmy.com/rat/viewtopic.php?t=11394 with a caption saying that it is "a small mask of a germanic face wearing a Suebian knot in the BM (Blaccus Collection). It had traces of red pigment on the face."
Because of the red paint, it is suggested that this is a (rare!) portrait of a Batavian. The same page quotes Martialis, the 1st century author: "I am a potters jest, the mask of a red faced Batavian. Though you make fun of it, a boy fears this face."
Looking at the photograph, the object seems to be on display at the British Museum. I will try to find out more when i go back there in May.
In the meantime, can anyone else now say something more... what, for instance, is the Blaccus Collection?
Thank you,
Kelvin Wilson