FX5200 TV-out aspect ratio problems

Hi, wonder if anyone can help.

I'm using an FX5200 connected to a Sony TV via S-video, which is working
well. I'm in the UK btw, so this is PAL we're talking about here.

The only problem I'm having is that the aspect ratio is slightly wrong.
A 4:3 picture which displays perfectly on the monitor is displayed
compressed slightly in the vertical plane on the TV.

All I can find in the control panel are tools to move the image around
and zoom in or out. If I use the zoom so that the image fits the screen
perfectly width-wise, then I have black at thetop and bottom.

I need to be able to stretch the image vertically to correct the
problem. Is there any way I can do this? I'm using a resolution of
720x576, which I presume is correct?

Any help very much appreciated.
--
Toby