If the Netgear router is a combo dsl modem+router, you should be able to reconfigure it in "bridge mode", which will demote it to a simple modem. The public IP is then passed to your D-Link router over its WAN port.
That said, after doing a little research, I get the impression Sky may have installed their own firmware to replace Netgear's, and so I'm not sure what features are still available, what's been changed, etc.
In the worst case, you can simply connect the D-Link to the Netgear, WAN to LAN, respectively. Just make sure each router is using different networks (e.g., 192.68.1.x and 192.168.2.x). Only downside is you’re double NAT’d, which complicates firewall management should you need to port forward to local services.