I've played with FME within MapInfo V16 64bit and found that I had to translate each GML file individually and prefix the filenames with the relevant OS tile e.g. SJ8545_TopographicArea in order to place all of the files together into the same folder. Once all of the tiles had been translated into tab files, I then used seamless tables to bring them all together for ease of loading into my workspace, one for TopoArea, one for TopoLine etc. However then came the issue of theming the layers in order to give them the usual OS look and feel, this I didn't have so much luck with I'm afraid, it was far too time consuming to add a theme and then select the colours to match the OS standard. I think this is probably why most people use third party OS data loaders such as OSM2MIF/MIFTAB, MapGML etc. as the GML gets translated and themed accordingly, unless there's another way of adding the theming information to the layers?