My now 10 year old Aviator 46mm ( every day wear) needed a small repair and service about a year ago - by a local service shop (top brands) costing several hundreds of GBP.
The watch still runs well but after I suspect a minor drop onto a wooden floor a few months ago it started running a bit slow and recently I noticed it was about 30-35 seconds slow per day which is too much to live with.
I decided not to go for another expensive service but to try a DIY regulation.
For a few pounds I purchesed a case back opening tool and after a little research decided it was pretty safe if I only touched the fine adjustment screw. On opening the case I found the adjustment screw was sitting about halfway towards "slow" so never having done this before I (carefully) cranked the screw as far as it would go towards "fast". I then put the caseback on, set the time to GMT website and checked the next day - watch was still running slow but now by less than a second!
That was 10 days ago - today the watch is running 9 seconds slow (since the test started) so approx 0.9 seconds per day - pretty happy with that!
Looks like beginners luck and hopefully it will stay this accurate for the foreseeable future.