I've ticked the "always use this IP address" box on the 320L line in my routers IP addresses page. Do you think this will this do what it needs to do?
Functionally speaking, that description sounds like the definition of
Reserving the IP address. Simply put, reserving the IP address forces the router to save (or reserve) the same IP address for a particular device. Permitting the router to manage IP addresses will avoid conflicts like the one you experienced earlier this week, and provide the added benefit of offering a single place to manage IP addresses for all of your networked devices. Using this approach, you will avoid assigning IP addresses that fall outside your routers valid IP range and avoid possible IP conflicts from two devices that want to use the same IP address.
Your router is designed to manage IP addresses, so unless there is a compelling reason to use a static IP address, there is no need to distribute that responsibility among your connected devices as this can easily lead to problems.