Confusion reigns again! It is either the CAA or the CAMO who recommend the ARC revalidation. If you're in the uncontrolled environment you can choose between finding a CAMO who will take you on for the occaison, or go direct to the CAA
If the CAA does the ARC, £25/500kg plus £122/hour is proposed.
If a CAMO does the ARC £25/500kg plus whatever their charge is.