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)