Before you start the installation process make sure you have updated the firmware for both the 300G+ and 614+ from the Dlink UK site (not from the US site). If you read the Dlink installation guides, you must change the IP address of the 614+ to 10.0.0.1 (both the 300G+ modem and the 614+ router have the same default IP address).
The way I set up my network was as follows:
The first step was to get a link established with the 300G+ modem and my computer. I configured my computer network adaptor card to have an IP address of 10.0.0.2. Upgraded the 300G+ firmware using the utility program from the Dlink UK site and configure the connection IP address of the 300G+ to be 10.0.0.1 (the same as the 614+ router).
The next step is to insert the 614+ router between the 300G+ modem and the computer network adaptor card. I re-configured the network adaptor card to have have an automatic configurable IP address. Configured the 614+ router with the settings given above and set the 614+ IP address to 10.0.0.1.
This set up procedure has worked for me and I have both the "wired" and "wireless" download connection links of 470/508kbs.
Dlink have a number of useful installation guides for the 614+ and the 614+/300G+ which are worth downloading.