.050" Allen key is the tool, I dug deep into my tool box and found one. 4 turrets removed ( I have 2 of these scopes) and I abandoned the idea of drilling and tapping in favour of slipping an 'o' ring into the wide groove on each turret body. A smear of grease on the inside wall of the turret knobs and the adjusters now have more friction whilst still retaining their positive click feel.
'O' ring size 22mm i.d. x 2.5mm section.
The outcome is that the knobs are now indicating zero on the marker scale and I am confident there is far less chance of accidental movement of the adjusters.