Richard, I've been having a rummage on my copious hard drives (4.5TBs worth) and I've found a bucket load of OS Land-Form PANORAMA Contours data (I think it's most of the UK, if not all of it). They are 20000m squares of dxf contour drawings . The only problem is they have a grid on them, so you have to open them up in a CAD program and remove the grid and grid text, save them, run them through something like DXF to XYZ, then run that output through something like Surfer or Quickgrid, then open that data in something like Global Mapper to get it out as a .ter file (though you might be able to skip the last part if Surfer can output a Geotiff) Problem being, there are 812 files making up 4.6GB of dxf data :-( I'm Going to give a random one a go now. If I get anything usable out of it I'll post the resulting file here in a bit :-)
Miles