http://www.dlink.com.au/Products.aspx?Sec=1&Sub1=14&Sub2=26&PID=364 You definitely have a DGS-3100-24TG! Maybe it's only sold in Australia?
Anyway the gateway has a 4-port hub built into it, and it can be multihomed, but I'm guessing even this won't provide true separation between networks, because they'll be linked via the hub in the gateway.
My thought was to have 2 links from the gateway to the DGS-3100-24TG, one connected to a port untagged on VID1, and one connected to a port untagged on VID2. I would then have my 2 Internet-only PCs on a separate subnet to the rest of the network, and have the gateway multihomed on the LAN so it has an IP address in both subnets and is the gateway for both. But as I said, even though the two networks would be on a different subnet, the two networks would still be physically connected by the hub in the gateway.
Ok so, I need a gateway that either supports 802.11Q or has 2 separate LAN ports that can't see each other, and has at least 2 WAN ports (we have 2 separate internet connections, one for main connectivity, one dedicated for a VPN to a remote office). Are you able to recommend any D-Link products that can achieve that?