I have done a fair bit of copyright law study as part of my degree, I could well be wrong but as far as I know the duration of the infringement has little or no bearing on the legality of the case (although obviously, if they had used them for longer the issue becomes more complex, but brief copyright infringement is no more legal than repeated or longstanding copyright infringement) so you are well within your rights to ask for the fee you are entitled to.
With regard to their "if it could be said to be use at all" comment, if they genuinely believed that the the fact that they instantly took them down when challenged clears them of liability they are either stupid or poorly advised.
Don't accept the two hundred quid, fight it.
A) They are worth more than that
B) Don't let "the man" get away with it