I tried to edit my previous post to add the following, but the system said I didn't have adequate permission - so please excuse my replying to my own post.
I've partially resolved this. It turned out that the Uplink sound driver was muted both for playback and record. To un-mute these, I used the following procedure:
Double-click the Volume system tray icon The Play Control panel opens
From the Options menu, choose Properties The Properties dialog box opens
From the Mixer list, choose Uplink then select the Playback radio button then click OK The playback properties for the Uplink driver are shown
Ensure Mute All and the individual Mute checkboxes are cleared
Repeat steps 2 to 4 for the Record properties
Skype now reacts to incoming and outgoing calls. However, no audio passes in either direction and Skype drops the calls after a few seconds. For outgoing calls, Skype shows a message asking me to check that my microphone is connected. I suspect that there's a codec issue. I've configured Asterisk to use GSM, G.711 uLaw, or G.711 ALaw but I don't know how to check which (if any) has been negotiated or even which codecs Uplink can use.