Thank you for your reply. I tried both combinations but neither worked.
The letter's red card only contained an 8 digit "unblocking key" so I used that.
On pressing the final # key it does not show on the screen (the #) and no positive unlocked message (or any message) appears. It might be ignoring it perhaps [?]
No 15 digit number is provided for unblocking, but at the top of the letter there is an "order number" (which is 15 digits). Would it be that? I wouldn't want to just randomly try it as apparently we only get 3 attempts!
If I understand correctly this procedure is to change networks. The old working SIM reports the network as T-Mobile. What is the new SIM network? If that's T-Mobile too then is the problem elsewhere (e.g. SIM not compatible with Nokia 100)?
Thanks for your help so far.