A tyre pressure monitoring or warning system comprising at least one receiver responsive to signals transmitted by transmitters associated with one or more tyres. The system including a memory which stores a list of identity tags captured by the receiver, and a list of associated confidence values for each identity tag indicative of the confidence that the stored identity tag corresponds to a transmitter that belongs to the vehicle. The list may be split into a member and a candidate list comprising those identity tags that are deemed to refer and that possibly refer respectively to transmitters associated with the vehicle respectively. Initiators may be provided to initiate transmission from the transmitters in order to locate the position of each tyre.