I downloaded the OS Terrain 50 dataset in ascii format, then unzipped all the folders and searched for all *.asc files. I then copied and pasted these into a new folder, so that they could be easily located when geoprocessing.
I then used the "Mosaic to new raster tool" in ArcMap 10.2 and selected all the asciis as the input rasters, saved to a new geodatabase, defined projection (BNG), pixel type (32 bit float) and number of bands (1) to match the original files. I left everything else as default options.
The resulting image has large areas missing in the south east of England (see below). When you zoom in beyond 1:750,000 the gaps disappear. Another strange thing is that when I removed the layer from the table of contents, then re-added it the whole dataset now only shows when zoomed in beyond 1:1,500,000 (and gaps still until 1:750,000).
I don't desperately need to show the data at national scale, however just curious as to what might be wrong and whether anyone can help. Thanks, Sarah