7_Driver - 3:17 pm on Apr 19, 2012 (gmt 0)
I think I'd contact them and ask politely for the discounted price, as a valued customer - since it's quite a lot, and such a short period of time.
It's quite possible they'll happily agree - especially if you point out you're about to place another order - so then there's no problem.
If they play hardball, THEN you can invoke distance selling regs, take your next order elsewhere or whatever. But give them the chance to do the right thing first.