In no ATC literary reference found there is a tolerance on speed. Flexibility therefore remain dubious. However, different modes in aircraft allow for more accurate speed control by the pilot, use them appropriately when speed control is requested, update the FMC in accordance with instructions.
OP's reference to speed tolerance of +10/-5 refers the values used by e.g. 737 auto throttle when to advise DRAG REQUIRED or THRUST REQUIRED, they are not the same as ATC speed tolerances, beware not to confuse these!
With Enhanced MODE S, ATC can see your actual groundspeed and IAS, so non-conformity can easily be observed and reported. If unable to comply, just advise what you can comply with. Many times ATC is more than happy to facilitate a deviation from their required if this would for instance require a landing gear to be dropped early in order to achieve their requested speed or flaps to be extended instead by crew instead of enabling them a clean configuration.
Extracts below:
CAP 493 Refers in SECTION 1: CHAPTER 13 Speed Control
1.1 Controllers may request pilots to increase/decrease speed in order to facilitate a safe and orderly flow of traffic. Speed adjustments should be limited to those necessary to establish and/or maintain a desired separation/deconfliction minima or spacing and should not be outside the speed ranges laid down in MATS Part 2. Pilots shall be advised when a speed control restriction is no longer required.
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1.3 At levels at or above FL280, speed adjustments for aircraft in the cruise should be expressed in multiples of 0.01 Mach. At levels below FL280, speed adjustments should be expressed in multiples of 10 kt based on indicated airspeed (IAS).
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1.5 Pilots will adhere to the speed (IAS or Mach Number) approved or assigned by ATC and will request ATC approval before making any changes thereto. Pilots of aircraft unable to maintain the approved or assigned speed (e.g. for aircraft performance reasons) will inform ATC as soon as possible. In such circumstances, controllers shall apply an alternative method to establish or maintain the desired separation/ deconfliction minima or spacing between the aircraft concerned.
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2.1 For aircraft at or above FL280 that have been cleared to descend to levels below FL280, speed adjustments may be based on IAS. However, controllers should be aware that pilots might not be able to immediately change to IAS, as the timing of this change is dependent on variable aircraft system factors.
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2.4 Controllers should endeavour to permit aircraft to operate in a clean configuration for as long as circumstances permit. Arriving aircraft below FL150 should not be allocated a speed of less than 210 kt (200 kt for propeller aircraft) unless within 20 track miles of the runway threshold,...
2.5 Speed control should not be applied to aircraft after passing a point 4 NM from the threshold on final approach.
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4.1 Speed control phraseology is listed within CAP 413. Additional information and guidance on speed terminology, relationships, and speed control techniques, is at Appendix F.
APPENDIX F - Speed Control Guidance refers in more detail to explain what speeds are commonly used and how they relate...