I've done it!
Me. Myself. On my own. Thank you Tina, Craig, Wayne, Nicholas and those lovely people at PC World. Didn't need you in the end.
I won't bore you with the technical details. Let's just say it involved downloading drivers from the Internet, unzipping files, moving folders about, telling the computer to get off its butt and go and look for stuff and install it now, this instant, otherwise I'm going to have a fit, etc. etc.
Update (Friday): The replacement disk, which arrived this morning and which I thought of using to install broadband on my laptop instead of downloading the whole thing again, is also missing an essential driver and miscellaneous bits of info. Wonder what the techies would have suggested I do today if I'd waited for them to solve my problem.