qgis - OS terrain 50: .vrt from zip files

I tried to follow what was suggested in How to use OS Terrain 50 in QGIS, but I got the following problem:

• in spite of being generally able to generate .vrt files using gdalbuildvrt from unzipped .asc files, when I try to do it using /vsizip/ the resulting .vrt seems corrupted;


• 
  

  it only contains NaN values, as can be seen from the following QGIS screenshot:

  
  

  

  
  

GDAL version: GDAL 1.11.2, released 2015/02/10

First few lines for a single .asc file input:

ncols 200
nrows 200
xllcorner 440000
yllcorner 1200000
cellsize 50
    -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9 -0.9

First few lines of the corresponding output:

  PROJCS["British_National_Grid",GEOGCS["GCS_OSGB_1936",DATUM["OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",400000],PARAMETER["False_Northing",-100000],PARAMETER["Central_Meridian",-2],PARAMETER["Scale_Factor",0.999601272],PARAMETER["Latitude_Of_Origin",49],UNIT["Meter",1]]
    4.4000000000000000e+05,  5.0000000000000000e+01,  0.0000000000000000e+00,  1.2100000000000000e+06,  0.0000000000000000e+00, -5.0000000000000000e+01
  
    
      /vsizip/hp40_OST50GRID_20150709.zip/HP40.asc
      1
      
      
      

    
  

Does anybody know what might cause it?