1. A method of providing access control for a user device comprising: processing input audio data using an access control module having a speech recogniser to produce index data representative of the likelihood of matches of portions of the input audio data to a set of predefined phones; searching the index data for a match to any one of a set of phone sequences corresponding to predefined threat words or phrases in order to detect any of the set of pre-defined threat words or phrases within the audio data; and initiating a threat detection response by the access control module on detection of one or more said threat words or phrases, by varying the ability of a user to access the audio data via the user device, wherein the set of pre-defined threat words or phrases are categorised into one or more categories according to a level of perceived offensiveness for the threat words or phrases, wherein a level of threat is determined for the threat words or phrases detected in the input audio data based on the category in which the threat word or phrase is categorised, wherein the threat detection response by the access control module, and therefore the ability of the user to access the audio data via the user device, is based on the determined level of threat, said method steps implemented on a data processor.