I was worried about dead-pixels, however I should be able to return a monitor under the distance selling regs, as long as it's fit for sale rather than trying to go along with the manufacturers "guidelines". I think.

Legally you wont be able to return it under the distance selling regs if you've used it, which clearly you'll have to do to find out if there are any dead pixels.
Are you sure about that - it seems highly unlikely? Under those regs you can return things that are not fit for purpose which for a lot of things you can only discover by using.
Yes, I am totally sure that returning something that is not fit
for purpose is not the same as returning it under the
distance selling regulations. I've had lengthy meetings with trading standards about this (and other legal issues) and have spent time with our solicitors to verify our position on this very issue.
I also have it here in writing (From Trading Standards) that our electrical products (Granted we don't sell monitors) are not suitable for return under the distance selling regulations if they have been used.
Different people at Trading Standards may give a different
opinion on this, which is why I requested that they seriously consider what they have advised me and that they put it in writing.
Of course a judge may have a different opinion if it went to court.