I confess to knowing little about Part M (being on the N-reg and maintaining to Part 91 requirements, with extras here and there) but today I spoke to the director of a small Part M firm (who I know personally) and mentioned to him some of the stuff mentioned here, and his reply was that these people are taking the p*ss and are a load of crooks.
However, this
Also no one at my home airfield objects to my working in the hangar.
is hugely exceptional. On the UK GA airfield-political scene, it is almost impossible to find a hangar in which you are allowed to do maintenance. In my hangar, I am allowed to use a hoover, maximum. A freelance maintenance bloke I know used to have such a hangar but it was a highly discreet arrangement and, for me, 1hr flying time away! So my 50hr checks are done outdoors; we have to pick a reasonable Sunday, and in the winter we stick our frozen fingers between the still-warm cylinders to keep warm. It is ghastly and makes me curse GA airfield politics and the people who wield the power in it.
And the local engineering facility are happy to allow me to use calibrated tools and test equipment.
That is feasible if you give them "some" work. I actually bought most tools needed (just ordered an oil filter cutter for £130) but normally to get a "relationship" like this you need to let somebody do the Annual, for example. This game is all about relationships.
But one has to start the right way from the very outset (at a particular location); if you get some firm to do the Annual and the 50hr checks, and then take away the latter and do them yourself, they will probably just tell you to p*ss off altogether.
Gasax is right about most firms being unprofessional.