OK, after doing a search i have found
this thread and it indicates that a set top box (which i have) is incompatible with a PS2 network adapter.
So, i either need to buy a £30 router, or clone my console MAC address to my PC and re-register it.
This is pretty tough going. I had no idea it'd be such a headache.
As i have asked in that thread i have linked to: How do i step by step go about cloning the MAC address?

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OK, after further digging
here, it appears my network card does not support cloning the MAC address, and the alternative method is to mess with registry settings, which i'm not sure i want to do.
So that leaves me the option of buying a router for £30. Ho-hum.