Hi guys, thanks for your replies thus far. Currently the ADSL router is set up to ford a port for RDP on the server. This is a consumer level router and doesn't provide VPN. Am I correct in thinking Remote Assistance would be a quick and easy way to allow remote administration with no changes needed to our current network. I have never used it myself, but like the fact that remote access must be acknowledged by someone onsite...
Setup Remote Assistance as others have mentioned and it should be the easy solution for your case, but do it over a PPTP VPN at least.
...On a side note, I understand having the ability to RDP into the server without the use of a VPN is probably asking for trouble. Replacing our current router with one that can provide VPN access is on my wish list. Brace yourselves for a newbie question, but do VPNs require capable hardware on one end with some sort of VPN software on the connecting side?
Most simple setups involve a VPN software client establishing a channel terminating at a hardware VPN server, which could be a dedicated box or all-in-one consumer router like the Asus I mentioned.
Windows versions since XP Pro have built-in VPN clients which connect to popular types of VPN hardware servers as well as commercial services. I use Windows 8's version to connect to the PPTP Server of an Asus RT-N16 (official firmware 3.004.260) at one of our offices. It's been reliable so far over consumer ADSL with RDP and shares, although file transfers driven by old industrial apps sometimes drop intermittently. I did reconfigure IPs to match the router's 192.168.10.x LAN addressing, to quickly ensure compatibility with any embedded routing scripts. One nice thing about Asus routers is that they come with an automated subscription to Asus's DDNS service. This allowed me to fix its url pain-free as the ADSL service of the site wasn't already on static IP.
Other VPN protocols like IPSEC are more secure but usually involve costly hardware. These may also require proprietary software clients for best performance (e.g. Cisco). I've been told that more expensive VPN stuff generally performs faster crypto for faster throughput, but this should concern you only if you're enjoying 10Mbps or faster connections.