Well, I collected my "weather PC" and have it installed in the attic with the WS2300 connected via serial for the last few hours. No strange readings yet even though I've had a connection dropout (more on that below), this time when the connection dropped ( there seems to be two ways the 2300 itself deals with this, sometimes the display shows OFL and others just -) the Cumulus display showed the last good reading, is that the way you wanted Steve?
When I started the fresh install of Cumulus on the weather PC (use data-logger was ticked by default) Cumulus downloaded the historic data from the station, but seemed to become a little confused because the monthly total reported from the station was higher than it seemed to expect, it was adding the difference into this hours total. I decided at that stage to make another clean install this time unticking use data-logger, and all seems to be working as expected.
I have the PC uploading to http://weather.kilmoneynet.com/
if anyone's interested.
PS. The WS2300 definitely doesn't like DECT phones, the attic where I put the weather PC (to keep it clear of the kids) also is home to my networking and VOIP gear including a DECT base station, it lost contact within 20 minutes and didn't reconnect till I pulled the power on the DECT setup.