Ordnance Survey Terrain 50 is distributed as a set of 10 km x 10 km tiles, each labeled geographically by a two-letter key (map here). There is a hierarchy to the data directory so that each tile is a directory, and within that directory there are a set of smaller tiles. I am going to assume that you just downloaded the entire dataset and have unzipped it somewhere, called
root_OS50_terrain. Within that directory there is a
data directory and
Let's say you only want the
tr 10km x 10km tile. It might be easiest to merge the ascii files into a single raster and then transform that merged dem into a hillshade.
$ cd root_OS50_terrain/data/tr # go into the tile directory
$ unzip '*.zip' -d tr_unzip # unzip all zip files into a new directory
$ gdal_merge.py tr_unzip/*.asc -o tr_tile.tif # merge all ascii files into a geotiff
$ gdaldem hillshade tr_tile.tif tr_tile_hillshade.tif
If you need a single raster representing all the two-letter tiles, then do this procedure (except hillshade) for each 10km tile, merge these DEMs, then hillshade that merged DEM.