Thanks, everyone.
OK - one wall socket will have the microfilter plugged into it with the filter's ADSL port connected to the ADSL/WLAN box and the 'normal' port connected to the DECT base. The other microfilter will be plugged into the other wall socket wilth thin air connected to the filter's ADSL port and a doubler connected to its 'normal' port to feed the fax/phone and Sky Rx.