I spoke to Planet X today, and they seemed alright about this, in fact they seemed as pissed off as I am about the likely cause of the damage, namely the couriers.
They suggested photographing the damage etc, but when I said I was bringing it in person, they said someone would be able to swap it for me on Saturday morning.
The slight glitch is that they don't have mechanics in on Saturdays, so any headset pressing will have to be done elsewhere, at my expense, of course. A small price to pay, though.
I'll be taking my wheel and some brand new bb cups to make sure any replacement is ok!
So there's still an outside chance of a ride at the weekend, but I'm not getting my hopes up to have them dashed (again).
As an extreme measure, if I can't fit my wheel into a painted frame, I might opt for a natural finish because I think the lacquer plus paint is a contributing factor to the tightness of the non-damaged left dropout.
On unbuilding the frame, I have found that I have not even damaged the paintwork with the seat clamp etc, so Planet X really have no cause to refuse me a replacement! From speaking to them, I am pretty sure they aren't going to stitch me up anyway.
Wish me luck.