Where are you in London. I had a steel frame retracked by Herne Hill Cycles and they did a good job. Very messed up rear triangle: niece rode in Cambodia and Laos, then flew back to Heathrow. No bike. Next day - 'where's my bike, BA?' - "In vancouver'. 'Right...when will it be coming back then...' 'Tomorrow'. Two days later 'where's my bike BA?' - 'In Chile, Santiago'. (You couldn't make it up), A week later, back in London with a completely mangled rear triangle. Brixton Bikes said 'you need a new frame' to my neice. I said 'it's salvageable...but don't watch them retrack it'. 'Why?' '....because 9 foot scaffolding poles are involved...and lots of welly....'
Two days later: perfect rear triangle, no paint flaking, all joints good, and that frame did thousands more miles in Asia, South America and the most hostile environment going, South London.
I've certainly filed dropouts in the past. But I would suggest that in this case - since you used 'Sheldon's String' I would go with re-tracking. Once filed, you can't put the metal back in the dropouts. Makes the frame lighter, though.