As promised I am back to tell the end of the history.
My MZ-N707 returned from England perfectly fixed and working fine.
The guy (Jim) has done a good work and a perfect feedback on what he did and why and how he did it.
I highly recommend him to anyone with a broken down MD recorder.
Ask him before sending a Hi-MD recorder. You'll find the list of the models he can fix on eBay.
Nothing more to say. Just that you can trust him