Couldn't PM you so here is my experience.
I sent in my W123 for a service in 2016 - he agreed that he would inspect/service/ repair all issues after conferring with me. The budget agreed to was "up to $2000." To be fair, the car's drive shaft had split causing him to breakdown enroute to Swansea.
A few days later, Cosovich calls only to insist that the car cannot be serviced and must be fully restored. He used colorful adjectives and peddled a strong sense of urgency to highlight that just repairing the car would be insufficient. After insisting that he stick to the agreed budget, Mark asks that I have my car recovered from his garage at my cost. He then threatens to charge me for inspection fees if I don't agree to restore the car with him. I should have run at that stage but I didn't.
I'm skipping over some of the gory details here, but after several agonizing calls, he eventually agreed to perform this full restoration for £8500 all inclusive. It was very difficult to lock him into the "all inclusive" term but he eventually conceded. I shudder to think of what would have happened if he hadn't. What he did not know is that I had priced the cost of labour and parts for the same job (minus powder coating etc.) which came up to around £5500. So I justified the added amount to myself because he would be bead blasting, powder coating and fixing a few additional smaller items all while providing a 1 year warranty for his work while he had the car in his care. (My W123 has SLS suspension, for which expertise is rare in the UK).
Fast forward to delivery of the car 6 months later, following completion of the work. I pointed out that the air-conditioning still did not work, the suspension made terrible noises over speed humps and a grinding noise continued to emanate from the rear. Mark grudgingly took it back and replaced the condenser as it was barely hanging on. He then proceeded to assure me that he would address the other suspension/ drivetrain noises in the new year. Mark continued to ask for more time and by this point, his 1 year warranty period had ended. Immediately after this, he highlighted that the car was no longer within warranty and that he would not repair the remaining issues at no cost. When I reminded him that he had assured me he would do this as part of the warranty in writing, he responded that I should be grateful that he did the job I had paid for, and that he had thrown in a lot of extras as part of that.
Frankly, I would have derived some satisfaction from taking him to small causes court over this, but the issues he neglected can be rectified for less than the hassle and cost of a case. I suspect but cannot prove that he may have realized this as well. I'll end by saying 3 things:
1) Mark epitomizes the following quote "empty vessels make the most noise"
2) You'll find the majority of praise comes from people who buy his book or who trust him blindly ( a colossal mistake)
3) If you want the same level of expertise for your W123, paired with absolute honesty, I'd strongly recommend
Holford Motors in Hornchurch. Paul is an ex-Mercedes tech who trained on the W123 when he was in his 20s. What's more, he's a gem of a person and has rectified all issues for me. Can't praise him enough!
Best of luck and let me know if you have any further questions about any of this!