I knew it! After splashing out for one of those Nintendo USB gizmos, I've discovered how to get the Nintendo DS to recognise the Conceptronic C54APRA router. In the end it just needed the updated firmware. Sigh. For anyone in a similar situation the firmware you need is this one: V2.00B02T02.GE.20051102 [conceptronic.net]
Don't forget to make a note of your settings before going ahead, since updating the firmware wipes everything. Once that's done, the DS will pick up the router when you scan for it in the wireless settings. Without the update, it's as if the access point didn't exist.