Thanks a bunch, all!

Now that I've set the aggressive XP as static (.99) and the others as dynamic, it's been ok, so not wanting to mess it up when it's fine, I'll leave it as it is.

Oddly, I downloaded the user manual for my ZyXEL and none of the pictures in it resemble what I have in the web configurator. The menus are wrong too. I assumed it was a firmware update, tried to update and the router wouldn't accept the .bin file, so no joy in that regard, but as I say - everything seems to be working fine so consider this solved.

Thanks all
D