I ordered my upgrade to ADSL from PlusNet and then saw on the forum all the problems people were having with USB Modems.
Undaunted (and trying to get Broadband installed as cheaply as possible ) I ordered an Actiontec external USB Home DSL modem from Dabs.com for under £20!
This afternoon, I got the e-mail from PlusNet to tell me my ADSL was enabled - 24 hours earlier than estimated (thanks, PlusNet/BT); opened up the Actiontec box, followed their installation guide, and here I am, on ADSL at last with no problem at all!!
Maybe I shouldn't gloat too early, but at the moment, here's one very happy PlusNet (and Dabs/Actiontec) customer - thanks to all concerned.