Getting a router which has had DD-WRT freshly installed on it is a lot simpler than you might imagine.
Virgin Media (appear to) use DHCP for assigning your router an IP address.
In Setup > Basic Setup, ensure that “Connection Type” is set to “Automatic Configuration - DHCP”.
You can leave all Optional Settings alone if you want to.
You should not have to set the Gateway value under Router IP, however you can try 62.31.162.1 if you’re running into problems - that’s the IP I’ve been given.
Getting it to work is simply a question of turning your devices on in the right order, just like setting it up normally. Unplug the modem and router, and wait for 30 seconds. Turn on the modem, wait a minute. Turn on the router, and connect to it from your computer. You should be connected to the Internet.
To check out your newly assigned IP address, which is sort-of confirmation that you’re connected to the net, go to Status > WAN and find it listed under “Configuration Type”.