Apologies for not getting back to you.
Is does appear to be the case that WAN ping response is not a feature of the router which is a bit of an oversight I think.
The only way to do this with the router is to route the ICMP traffic through to a PC and get the PC to respond.
This means if the end user want's to do something like quality monitoring of their broadband link using a website which continuously pings their IP then they must route the traffic and leave their PC on all the time.