Iain,
I live near Spalding in Lincolnshire and like you I have both the WMR100 and 200. Mine too show different pressure readings, I have found the 200 more accurate compared with other stations at the same altitude and within a ten mile radius, it doesn't bother me, I just live with it. The only way you will get the 100 to play ball is to adjust the altitude accordingly, but as I only use the 100 as a stand-alone unit and not for displaying data on the Internet, this is not a problem for me.
Hope this helps.
Alan