Surgical instrument radio frequency identification, RFID, tag reading apparatus (10, 30, 40, 50, 60, 70, 80, 90) comprising: an interrogation zone arranged to receive a surgical instrument set comprising a plurality of surgical instruments, each surgical instrument comprising a respective RFID tag containing identification information for the surgical instrument; an RFID tag reader; a radio frequency, RF, antenna coupled to the RFID tag reader and arranged to transmit an RF signal having a signal field at least part of which extends within the interrogation zone and arranged to receive RF return signals from at least some of the RFID tags, each RF return signal containing the identification information of the respective RFID tag; guidance apparatus (22, 32, 42, 52, 62, 64, 72, 82) arranged to cause a predetermined relative movement between the RFID tags on the surgical instruments and the RF signal; and a controller arranged to: a. receive the surgical instrument identification information from the RFID tag reader for each RFID tag from which an RF return signal is received; b. compare the received surgical instrument identification information with a surgical instrument list of the surgical instrument set; c. determine whether surgical instrument identification information has been received for each of the surgical instruments on the surgical instrument list, and if not: generate an incomplete list alert, and characterised in that the guidance apparatus is arranged to cause a predetermined rotational movement between the RF antenna and the surgical instrument set and the controller is further arranged to, in response to an incomplete list alert: d. transmit a control signal to the guidance apparatus comprising instructions arranged to cause a predetermined rotational relative movement between the RFID tags on the surgical instruments and the RF antenna, the predetermined rotational relative movement being different to said predetermined relative movement; and e. repeat a. to c. an interrogation zone arranged to receive a surgical instrument set comprising a plurality of surgical instruments, each surgical instrument comprising a respective RFID tag containing identification information for the surgical instrument; an RFID tag reader; a radio frequency, RF, antenna coupled to the RFID tag reader and arranged to transmit an RF signal having a signal field at least part of which extends within the interrogation zone and arranged to receive RF return signals from at least some of the RFID tags, each RF return signal containing the identification information of the respective RFID tag; guidance apparatus (22, 32, 42, 52, 62, 64, 72, 82) arranged to cause a predetermined relative movement between the RFID tags on the surgical instruments and the RF signal; and a controller arranged to: a. receive the surgical instrument identification information from the RFID tag reader for each RFID tag from which an RF return signal is received; b. compare the received surgical instrument identification information with a surgical instrument list of the surgical instrument set; c. determine whether surgical instrument identification information has been received for each of the surgical instruments on the surgical instrument list, and if not: generate an incomplete list alert, a. receive the surgical instrument identification information from the RFID tag reader for each RFID tag from which an RF return signal is received; b. compare the received surgical instrument identification information with a surgical instrument list of the surgical instrument set; c. determine whether surgical instrument identification information has been received for each of the surgical instruments on the surgical instrument list, and if not: generate an incomplete list alert, and characterised in that the guidance apparatus is arranged to cause a predetermined rotational movement between the RF antenna and the surgical instrument set and the controller is further arranged to, in response to an incomplete list alert: d. transmit a control signal to the guidance apparatus comprising instructions arranged to cause a predetermined rotational relative movement between the RFID tags on the surgical instruments and the RF antenna, the predetermined rotational relative movement being different to said predetermined relative movement; and e. repeat a. to c. d. transmit a control signal to the guidance apparatus comprising instructions arranged to cause a predetermined rotational relative movement between the RFID tags on the surgical instruments and the RF antenna, the predetermined rotational relative movement being different to said predetermined relative movement; and e. repeat a. to c.