the time frame is important as the op did not really agree to pay over the span of a year.
If you bought a car they charged you half no then half at a random point in the future you'd be a bit miffed as depending on your finances a whack of cash lost at the wrong time would be very inconvenient.
Ok, irrelevant was a bad choice of word, unimportant would have worked better because 14 months is fully acceptable under the law. As for the car example you would have nobody but yourself to blame if that happened because you would have to be stupid not to notice they only took half the money, its an easier mistake to make with £40 obviously but still, he owes money for something he received and is legally bound to pay it.