Based on 2010-2011 data, this BIS study (which I've not read) shows that it is not uncommon for adults who have already got, say, Maths GCSE (or equivalent) at Grade C or above to enrol on a numeracy course at a lower level. This probably strengthens our hunch that there are more people "out there" who might gain from CM than would be assumed from the proportion of adults who have already got a Level 2 Maths qualification.