pasty boy,
Regarding tolerance, as an example, from the UK AIP entry for LHR...
Speed Control: Pilots should typically expect the following speed restrictions to be enforced: 220 kt from the holding facility during the
initial approach phase; 180 kt on base leg/closing heading to final approach; between 180 kt and 160 kt when established on final
approach and thereafter 160 kt to 4 DME. These speeds are applied for ATC separation purposes and are mandatory. In the event of a
new (non-speed related) ATC instruction being issued (eg an instruction to descend on ILS) pilots shall continue to maintain the previously
allocated speed. All speed restrictions are to be flown as accurately as possible. Aircraft unable to conform to these speeds must inform
ATC and state what speeds can be used. In the interests of accurate spacing, pilots are requested to comply with speed adjustments as
promptly as is feasible within their own operational constraints. Pilots should advise ATC if circumstances necessitate a change of speed
for aircraft performance reasons.