Thanks to all for the replies. Funnily enough (actually, I didn't find it so funny) I got home to find my wife had unplugged the Sky hub to plug something else in. It was slightly ironic as had the ECI been connected still, the WAN wouldn't have dropped. So I thought it might be a good opportunity to get the ECI reconnected. The ECI was connected (DSL light steady) but the Sky hub just wouldn't connect in WANoE mode. I reset it several times, but it was a no go, despite being setup and working this way previously for some time. I half wondered if the exchange fault with Sky's equipment had emerged again, but no; the Sky hub connected directly to the phone line (i.e. in modem/router mode) worked just fine.
What gives I wonder?