Yep, tried Robin's page and tried most of what seems to apply. I can see that son can't have put correct MAC address into the router because he didn't keep a note of the MAC address for either machine before he started, (d'oh - but then he's the one who did computer studies at A level, hence my naive trust!)
Even with all his fiddling around, I can't see that he could have cloned another MAC address into the NIC on either PC so I can only assume that what shows up in ipconfig/all is what it was before the router installation.
Either way, the STB has recognised the MAC's previously for both machines when they have been the sole machine using broadband. It concerns me that son's machine couldn't subsequently access broadband after we'd set up the Evesham machine. I see from Robin's web-page that the NTL server use to only recognise one MAC, but as the page goes on to say, that is no longer the case, and Tech Support have confirmed this. However, I have followed Robin's suggested procedure for de-installing one computer and replacing it with another - all to no avail.
The complicating factor from my perspective is that neither machine can now be set up with broadband - it seems too much of a co-incidence that there would be the same hardware glitch on both.
It's a pain in the A*** - every time I try something I have to break the dial-up network sharing that I've set up, turn off set-top boxes, etc, all with three kids and a wife who either want to be on the internet or watch cable tv breathing down my neck!
My usage bill came to £150 extra last month plus a rental for a BB connection that I can't connect to! I've even tried getting local firms out to come and have a look - as soon as I tell them its NTL with a set-top box they run a mile.........