D-Link DSL-500 remote access - how?

It sounds like you're using VPN (Virtual Private Networking) with your router. You should be able to find a spot to allow "VPN Passthrough" in the router configuration.
If it asks for anything else you'll have to hunt that information down.

Riser

Thanks Riser - managed to get it working eventually. Any network admins will probably shake their heads reading what I did, if it's wrong let me know please!

Within "NAT Configuration" I enabled DMZ and set the IP to redirect traffic to to 192.168.0.2 (the network address of the server's internal network connection).

That works, just as long as you know that your 192.168.0.2 machine is no longer protected from the outside world. You should have firewall software installed on it, but then you will have to configure the firewall software to allow your remote access.

If this is a server or something that has shares open for employees on the network, then those are also open to anyone on the internet.