There is a bit in ICAO Doc4444 which mentions speed tolerances, but then throws it back to the relevant ATS authorities to specify them 
Use of Radar in Air Traffic Control Service - Functions
7.1.1 g) maintain radar monitoring of air traffic;
Note.— Where tolerances regarding such matters as adherence to track, speed or time have been prescribed by the appropriate ATS authority, deviations are not considered significant until such tolerances are exceeded.
No specific figure for tolerance published = no tolerance or leeway in my book. 
Anyone able to point out that the CAA do in fact publish a tolerance level for the UK ?? All I can find is the stuff in AIC 53/2004, which is a straight lift from text in ICAO Doc 4444 again, namely:
4. Tactical Application Of Speed Control By ATC
4.1 In busy traffic situations it is necessary for ATC to limit aircraft speeds for traffic sequencing purposes and reduce the need for aircraft vectoring. Controllers will consider aircraft performance limitations in issuing speed instructions. Specific speeds should normally be expressed in multiples of 10Kts.
4.2 Only minor speed adjustments of not more than +/- 20Kts should be requested of an aircraft established on intermediate or final approach.Speed control shall not be applied within 4Nm of the threshold on final approach.
4.3 Aircraft unable to conform to the speeds specified by the radar controller should inform the controller immediately and state the speeds which shall be used.