Vision 4204 Plasma, top 2" of picture missing

[spareroom]

Hi people, I recently bought a Vision 4204 Plasma screen which in most respects is working perfectly apart from one small detail. The top inch and a half or so of the picture is blank, cutting off the very tops of peoples heads on close-ups. 2" Horizontal line?
I've taken a breif look at the screen and to be fair, the pixels appear to be live in that they appear to be part od a working screen, but the picture isn't extending to the top.
Any ideas?
Tony