Those of us who give our services as instructors and coaches freely and without payment are classed as volunteers and the CRB makes no charge in our case.
Correct, but that is not the whole story. For a voluntary organisation to obtain CRB checks certain systems must be in place to ensure the proper handling and administration of the checks. For large bodies; e.g. The Scout Association; who process thousands of enquiries a year it is feasible to process the checks in house and absorb the cost in the administration budget. By comparison the NSRA obtains a handful of CRB checks each year and the cheapest way to obtain and process them is via a 3rd party provider, in their case DMG.
Whilst Disclosure England/Scotland may provide the information free of charge, obtaining a CRB check is far from being a no cost exercise.