Thanks for the info and opinions.
The current modem was supplied by TT six years ago and uses an ethernet connection, which is what the new one would be set up for since I don't currently need wifi.
The one they've sent is a Huawei HG533, if that means anything to anyone, and the spiel says it should be kept on so it remembers my setup and for "enhanced remote management" (so they can fiddle with it).
I get the impression that I could just unplug the existing and put the plugs in appropriate sockets, after swapping the psu.
Where did anyone get the idea I am using usb??