Correct NVM chip for a Scantronic 9827?

After replacing the battery recently I needed to reprogram the system. I don't have the engineer code so followed the instructions for removing / resetting the NVM chip. After doing this several times however the user and engineer codes are not being reset to the defaults. My existing user code continues to work to silence the alarm after restoring power. I have read that the NVM chips have a limited life (mine is 15+ years old) so wanted to have a go at replacing this before going any further as its a cheap fix if it works. I have read that the chip is an NVM 93C46 but on the Farnell website there are a number with this code. (eg. AT93C46D-PU, AT93C46E-PU, 93C46B/P). Can anyone tell me please which is the correct one? Your advice would be much appreciated. Thanks.