although this didn't work yesterday, when I came to try again today, the wireless clients worked fine. It looks like there is a timing delay? Or maybe themodem-router rebooted overnight and that enabled the ap to get the info?

[update]
So we've now discovered that having this device operate as an AP in Smart IP mode on an existing modem-router with its own DHCP server on the 10.1 network was causing chaos on our home network because both the AP and the M-R were serving up DHCP addresses; this didn't make sense given that the WA830 DHCP server was disabled - but that's what the network diagnostics told us.

We then setup the WA830 AP with a static ip on 10.1.1.253 and disabled Smart IP mode. Its now working tickety-boo and reliably.

The learning? Don't assume the doco is perfect. Lacking confidence, I needed confirmation from a more experienced person that my gut sense I should use a static IP on the AP was in fact the right choice, even though the doco seems to suggest using Smart IP is OK.