Just to add my experiences as a kind of 'joker' amongst UK based aircraft owners.
Part M is nearly the same as the French system which I have used for 10 years on my French registered aircraft. Under that system a 3 yearly CDN airworthiness review took a mornings work and cost around 200 Euros. Now under EASA part M the same airworthiness review is called an ARC in English and a CEN in French. The review can be done on any EASA aircraft by any CAMO (with the required approvals) in any member state. So far the reaction of UK CAMOs to my request for an airworthiness review on my EASA aircraft has been comical. First they are aghast that I expect them to look at the documentation only as far back as the last review. Second that I do not require any maintenance and have myself carried out and released the last three annuals under French rules. Thirdly since I know exactly how much work is involved and how long it should take, I expect and insist on the bill not exceeding £300. After 28th Sept 2010 I will no longer be able to release my own annuals, but by that time a number of French CAMOs will be up and operating offering ARC/CEN renewals at a fraction of the current rip off UK fees.