geotiff tiff - Geoserver not displaying mosaic TIFs

I have an OS dataset loaded into Geoserver as an 'ImageMosaic'. There are 10588 individual GeoTIFFs.

Geoserver obviously recognises and reads the GeoTIFFs, because if I try 'Layer Preview', it gives me a preview with the correct extents (both in OpenLayers and KML/Google Earth). However, there is no data displayed.

I have also tried loading a single GeoTIFF. The result is the same.

It should not be a problem with the GeoTIFFs themselves, as they open without any problem in QGIS etc., and they are official OS data, so you would assume they are good...

Does anyone know why my rasters are invisible?

GDAL info output:

Driver: GTiff/GeoTIFF
Files: TV69NW.tif
Size is 5000, 5000

Coordinate System is:
PROJCS["OSGB 1936 / British National Grid",
    GEOGCS["OSGB 1936",
        DATUM["OSGB_1936",
            SPHEROID["Airy 1830",6377563.396,299.3249646000044,
                AUTHORITY["EPSG","7001"]],
            TOWGS84[446.448,-125.157,542.06,0.15,0.247,0.842,-20.489],
            AUTHORITY["EPSG","6277"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],

        AUTHORITY["EPSG","4277"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",49],
    PARAMETER["central_meridian",-2],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",-100000],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","27700"]]

Origin = (560000.000000000000000,100000.000000000000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Metadata:
  AREA_OR_POINT=Area
  TIFFTAG_COPYRIGHT=▒ Crown copyright and/or database right 2016 OS
  TIFFTAG_IMAGEDESCRIPTION=OS Open Map - Local - Full colour raster - TILE TV69NW
  TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
  TIFFTAG_XRESOLUTION=254
  TIFFTAG_YRESOLUTION=254
Image Structure Metadata:

  COMPRESSION=LZW
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (  560000.000,  100000.000) (  0d16'10.78"E, 50d46'36.88"N)
Lower Left  (  560000.000,   95000.000) (  0d16' 2.96"E, 50d43'55.12"N)
Upper Right (  565000.000,  100000.000) (  0d20'25.89"E, 50d46'31.84"N)
Lower Right (  565000.000,   95000.000) (  0d20'17.82"E, 50d43'50.09"N)
Center      (  562500.000,   97500.000) (  0d18'14.36"E, 50d45'13.50"N)
Band 1 Block=5000x32 Type=Byte, ColorInterp=Palette
  Color Table (RGB with 256 entries)

    0: 0,0,0,255
    1: 8,7,7,255
    2: 24,24,24,255
    3: 40,40,40,255
    4: 52,52,52,255
    5: 85,15,45,255
etc...