When i was going to CTC assessment a long long time ago. I did a bit of research about the tests and actually wrote a computer programme which was designed to simulate that nasty super duper multitasking one at the end. I got it so it did the numbers thing as well as the shape thing similtaneously. Though you didn't use the keyboard to tap it in, instead i just acknowledged the number in the shape when it turned up and when the number pattern changed. Then to finish the effect i did a level on Pilotwings on the N64 at the same time, that or that ridiculous helicopter game you find on the internet. Must say, it was startingly accurate to the one i was presented with on the day, even the Mrs Hawkins voice.
I dunno how much i benefitted from that, it certainly gave me more confidence though even still i made a few cock ups on the day. I managed to pass though.
Frankly though even if it increases your chances only 1% they (the prep software) are still probably worth getting, because presumably a potential job may ride on that test so you need to maximise your chances and that 20 quid is nothing given the amount of training you have done/will do.