It appears to say its for ignoring the Red X and the accompanying image confirms it??!!
They've not referenced the speed in terms of the prosecution.
You must reply and name the driver at the time/date and then s/he will get a further letter. Unless you've a miracle reason, you're bang to rights (or, at least the drive ris!). Others will correct, but Idon't think there's a course offer for this so it will be fine/points.