Brian,
to CF.
Pretty much correct.
You need an RJ45 cable from STB to router WAN port. You then need another RJ45 from a router 'client' port to your PC (or USB adapter). Most routers will come with an RJ45 cable. You will get better performance if you add a £10 LAN card to your PC, but that assumes you are comfortable opening the PC, installing the card and that there is room in the system.
Note - you will need to powercycle the STB when you put the router in place. You will also need to register the router first time it's plugged in -
details here.
HTH