How do I identify the switch connected cable in an unmarked, disconnected ceiling rose amongst three cables. I believe that this can be achieved with a voltage testing screwdriver. Access to cables is limited in loft.

What is testing procedure?

The Tester not only needs to show a hot. But 120 Volts between 2 wires.
Identify the 3 cables(a,b,c or 1,2,3)or something and take note of existing connections.
May have to disconnect wires to get non confusing readings(backfeeding).
See which 1 has Voltage, the other 2(1 may go to another outlet, or carry power on, and the 3rd may be the switch. If you can Identify the 3 cables, we can tell you how to connect. Good Luck.

Personal Opinion: You would be better served by investing in a Volt/Ohm meter. This will allow you to test for the proper configuration with both voltage (Determine main power source for fixture). Voltage and/or Ohms for the operation of the switch.

Would you please consider appending a picture of the junction box wiring for us?