1 Open QGIS with OS DEM data loaded
2 QGIS->Raster->Extraction->Clipper save area as new file (.tif)
3 QGIS new project, drag in the .tif
4 Select the tif (DEM) data and then QGIS->Raster->Extraction->Contours
5 Select the new contours file and right click Properties. Instead of Single Symbol select rule-based. Load in the 3 colour contour rule.
6 Save as DXF (use correct scaling)