I tried using a Draytek Vigor, and a Belkin router, and an Apple Time Capsule to extend the WLAN/LAN of the D-Link DIR-615 - to no avail. I found out that the older Draytek doesn't support it, but the other two should, but wouldn't work. Seems to me to be a compatibility thing. Each manufacturer only tests against their own kit.
Upshot is that I got an Apple Airport Extreme, and have ditched the D-Link DIR-615. All working fine, no drops, high speed. Real easy to setup with the Apple kit.
So, to summarize:
Downstairs... WAN in --> Virgin coax cable modem --> Apple Airport Extreme WAN port (firewall, router, NAT) configured to allow WLAN extension --> (over air to upstairs) --> Apple Time Capsule (which incoporates an Airport Extreme) LAN port --> cabled to an eight port switch --> with several devices hanging off.
So all devices effectively in one LAN, i.e. the Airport Extreme downstairs acts as the DHCP server to all devices upstairs - plus I now have extended wireless range.
Simples.