Thanks for the offer, but I had a call from my ISP this morning and I've now got it working. In fact, the laptop is charging while connected wirelessly and the internet is still up! It must have been some sort of interference travelling along the wire. Now I can take the laptop to the phone if I need the ISP's help with any more debugging [I don't have a mobile, for not-only-Luddite reasons]. Hurrah!
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