I am without doubt the biggest foxtrotwit on the planet when it comes to maths believe me , im so bad they wouldnt even enter me for maths GCE not this poncy GCSE thing , thats how old I am.
I have succesfully completed the atpls with very little understanding of maths , I can just about understand how to rearrange a very simple formula and my arithmetic is almost juvenile . I used the oxford CD and found it to of absolutely no help whatsoever .It confused me teaching me things that thoughout the entire course I have never encountered (yes I was on a full time very well run GS course) I would save your money
The part that hepled me the most was that discussed by Mr squid above , the initial week when basic formula CRP/5 work etc etc is all covered and a bit of ppl brush up , I believe it is pretty much the same whichever school you attend , but I wouldnt spend money on a maths brush up CD I did and it helped me pass the exams in no way at all . Admittedly had I understood it etc etc my groundschool may well have been a bit easier , but I made it through , almost unscathed , so it is possible .
Good luck