Sounds like a bit of a mess, CJ, and I'm not sure there's much comfort to be had since the dual-boot set-up you tried to install may have invalidated the warranty.
Having said that, you should have been able to perform the built-in Full System Restore if the hardware was still intact. So I would be inclined to confront the Dixons/PSG organisation, if necessary with the help of the Trading Standards Officer or the Office of Fair Trading.
Try rattling a few sabres; you might get somewhere.
As for accessing a hidden partition, what about a Recovery floppy disk created on another machine?
BTW, when you created the dual boot setups on the earlier machines, did you have XP before 98? I think it would normally be better (or even essential) to install 98 first and then the NT system (be it NT4, 2000 or XP).
Hope that you get a satisfactory solution.