Having problems creating VRT in QGIS from png files

This is similar to a question asked by someone else, re OS tiles, however, there is a difference! I am a bit of a amateur in GIS and python, but if I take it slow I can usually just about work it out!

This one though: I have a large amount(over 10k) of geo-refd png files which I have mosaicked in Arc, but also want to be able to access them in other programs such as QGIS over the web and within a private network using Geoserver - so I have tried creating VRT's with them, and sometimes it works, but sometimes it doesn't: it seems that on selecting over 200-odd png's, the vrt stops being created: is there a limit on the amount of rasters for a VRT, or can some-one give me some help please!! This is so frustrating ;o) The error message is: "The process failed to start. Either the invoked program is missing, or you may have insufficient permissions to invoke the program."

and the code is:

    gdalbuildvrt -allow_projection_difference 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/SX.vrt" 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/1.png" 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/2.png" 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/3.png" 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/4.png" 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/5.png" 
    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/6.png" 

    "F:/Historic Buildings/GIS/Data/Mapping/Landranger/Raster/15k/Data/SX/7.png"

with an additional +200 similarly addressed png's

I am using QGIS 1.8.0 Lisboa, on W7 64bit

Any help would be gratefully received