T42S factory reset without default password

[JeffWilkinson]

I recently picked up several 2nd hand Yealink T42S handsets from auction.
These have been customised and the default password is not "admin".
I need to factory reset or change the password back to "admin", so I can login and reprovision them.

The firmware currently installed shows version: 66.81.193.110

So far I have tried the normal factory reset method of holding the "Ok" button down waiting for the prompt and then selecting Yes. This prompts for a password which is not "admin" (keeps saying wrong password).
I have also tried the another method I came across when searching for a answer - which is to hold the "speaker" button down and power cycle the handset. I get to the 1.TFTP or 2.USB display on the screen. I stop pressing the speaker button and press 1 for TFTP... It then displays some IP address settings (192.168.0.100,255.255.255.0,192.168.0.1 and a TFTP address of 192.168.0.23) . The instruction i had found suggested I press OK button and then power reset the phone and it would perform a factory reset.
Unfortunately it just boots back up with the same settings.
Can anyone help me ? I would hate to have to throw all these handsets out.