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As I see this the biggest problem with Part M is if you have a disagreement with your maintainence company simply because it will cost a lot to come out of the "controled enviroment" due to all the CAA fees that will be required to issue the ARC (annualy).
It may well be best for owners to use a Part G & I company to do the paperwork and another Part F company to do the maintenance work.
This may seem a bit complicated but for a relitively small fee it will keep the aircraft within the "controled enviroment" and at the same time let the owner shop around for the aircraft maintenance work.
The advantage of this is that the aircraft owner won't be "over a Barrel" when it comes to the expensive part of the maintenance and if the Part F company knows that the owner can move the work away without leaving the "controled enviroment" they may well be a lot more carefull about the way they treat the owner.
Owner maintenance is part of the Part F remit and as I see it the Part F company has first to make sure that the owner is able to carry out the 50 hour check to a satisfactory standard, they would then in effect issue the owner with a Company approval to do the 50 hour check or any other simple task that they see fit.
I can see this being a cause of much friction as there are some owners that I would not let work on a roller skate and others that work to a very high standard who I would be more that happy to let work to the full limits of the LAMP owner maintenance requirments. As "checking out" an owner would take time I would expect companys to charge for this training/checking before issuing an aproval for the owner to do 50 hour checks.