In England there are 71,500 Registered Childminders (\link{http://www.ofsted.gov.uk/publications/index.cfm?fuseaction=pubs.displayfile&id=4148&type=pdf\Source: Quarterly Childcare Statistics, Dec 2005})
NCMA has 50,000 members, according to Liz Bayram - figure from their March 3rd press release, titled NCMA voted Top 100 Company by Sunday Times. Alas can't link to this, as NCMA's website doesn't seem to like pages within it being linked.
So NCMA represent about 70% of childminders. However the figure is not fully accruate, as NCMA represents childminders in England and Wales, and the figure from Ofsted for number of Registered Childminders is only for England. But it gives a good indication of the level of support which NCMA have.
Childminders cost the government £750 when they register, and when they are inspected. \link{http://www.dfes.gov.uk/consultations/downloadableDocs/Partial%20RIA%20Part%202%20-%20Regulation%20and%20Inspection.doc\Source}
\link{http://www.publications.parliament.uk/pa/cm200506/cmstand/d/st051220/am/51220s01.htm\Another Source}
Childminders currently pay £14 for registration and £11 renewal.
The Partial Regulatory Impact Assessment (see first source above) estimates that Childminders cost £32 Million per year. This excludes the cost of dealing with complaints and follow-up visits. It also excludes Training and other local authority funding.
If the NCMA didn't campaign on behalf of childminders, I'm sure the government would charge childminders more to register and renew their certificates. After all, the government are charging more for nannies to register under the voluntarry Childcare Approval Scheme.