Mrf is Sagemcom and model is F@ST2504n. My ISP is Sky Broadband (in the UK) and the router is configured with their own settings.
Well, I've only noticed it change a couple of times, when I've logged on to the FTP service. They were a couple of days apart. However, I can't be certain how many times it's changed in between these times.
It's not really a problem (or I don't think it is) as it's only swapped between two different WAN IP's and so I just make a note of both so if one doesn't work, the other one will. I'll have to let anyone else who I set up as users, that they may need to try both.