When I went through the initial selection tests, I later found out that I too got a low (3/10) score for mental arithmetic, but was told that I was given the job because of my high scoring in spacial awareness and complex reasoning.
However, I took the tests before september 11th when NATS was having a hugh recruitment drive and they let more people in and put the students on watches down at CATC.
I was informed by HR at CATC that the way they work it is they have a "special" scale that ranges from one to ten in each section of questions.
They then have set bands within that scale which they say the majority of average ATCO's fall in between and if your scores from these "special" scales fall within the average ATCO criteria you'll get the job.
They also take the answers from your personality questionaire and to be honest, they do actually paint a very accurate picture of the sort of person you are, and they see wether you fit into the average personality trait of the average ATCO.
They don't seem to be looking for superstars but just mister average who can do the job.
You must remember though, that even if you get through these initial selection test and get to the college for training, you have to work bloody hard for up to two years before you even get to your unit, your constantly reminded that you are a mobile grade and could be posted anywhere or sent anywhere on OJT so if you haven't got a good enough excuse prepare to move or travel long distances, if your face doesn't fit ...... , but other than that, once you get past the first three to four years you'll enjoy life as a controller to it's full potential.
OR you could get a job as an ATSA at the airport you really want to work at that is non-NATS and with hard work and good time keeping etc, be sent by them on courses and become a controller that way. NATS is good because it's free training and they pay you but the other way works just the same and you'll know where you'll be working at the end of it and have an extremely good knowledge of the aerodrome before you're trained and go back for validation.