The map below shows the number of species recorded in each 10km square (according to my copy on MapMate at the moment) since 2004. I've highlighted six squares in red as to me these figures seem on the low side, and could be useful areas to look at trapping for the purposes of square bashing.
Incidentally, I did not mean to imply that I did not think the squares on our northern boundaries were not worth trapping, merely that without checking with Norman how well these squares are covered from his side of the boundary. I suspect not very well, but you never know!
In spite of what I said in my comment below, I've looked at the SEWBReC database and updated the outlier figures:
This so SO11 is as much need of further recording compared to some of our more central squares - but don't let that stop you getting out to them if the mood takes you!
And following George's request for macros only:
This is taken solely from the SEWBReC database, which should be up to date, but there may be some slight discrepancies from a MapMate produced equivalent. It still seems to indicate the same areas are not well recorded, but perhaps the SS99 and ST09 are not as badly off as first appeared!