The UM land-sea mask is a logical mask with LAND=TRUE and OCEAN=FALSE. The UM requires there to be at least one land point in the mask. One way to get over this whilst maintaining zonal and hemispheric symmetry is to put land in every point in both polar rows. This is the case in the land-sea mask provided in the tar file which can be downloaded from these pages.
The land-sea mask provided in the tar file is called aqua.mask