Hi Ben,
I think the problem is NumOfCellsPerLine, you have 5192. I have repeated the download (at resolution 1) and I get 6357:
[Source1]
Type = BMP
Layer = Imagery
SourceDir = .
SourceFile = Area_Lp1_SnapOff_N054382751_N054360809_W005563474_W005510742.bmp
Lon = -5.94298839569092 ;for top left and bottom right is: -5.85205078125
Lat = 54.6409857273102 ;for top left and bottom right is: 54.6022439002991
NumOfCellsPerLine = 6357 ;Pixel is not used in FSX
NumOfLines = 3611 ;Pixel is not used in FSX
CellXdimensionDeg = 1.43051147460938E-05
CellYdimensionDeg = 1.07288360595703E-05
PixelIsPoint = 0
SamplingMethod = Point
Channel_BlendMask = 2.0
Channel_LandWaterMask = 3.0
I did a quick and dirty blend and water mask and this is what I see when dragging the bgl onto TmfViewer from the SDK:
In fact you could have downloaded at resolution 0, however, this may have produced four bmps.