I called up T-mobile and they sent me a Network / SIM unlock code via email after asking nicely.
So it is true. You can request T-mobile supply an unlock code after being a customer for 3 months. That's great!
Now when I travel abroad I can use a local SIM. 
Thank you T-mobile for being sensible about this. This is the text version of the attachment I received:
Mobile number: Reference: DOX/CAT/ 2008 Dear Thanks for your letter about unlocking your MDA ****. The unlocking code for IMEI XXXXXXXXXXX is XXXXXXXXXXX and the sequence is below. 1. Switch on the mobile phone with a UK non T-Mobile SIM. 2. You will be prompted to enter the pin, after a few minutes. 3. Then enter the pin code. 4. Press OK. The phone should now acknowledge the phone is unlocked. We always advise that you shouldn’t make more than two attempts to unlock the phone. Therefore, if you get any error messages at the end of the unlocking sequence, please give me a call on the number overleaf and I’ll certainly help you. I’m sure you can appreciate that as our codes are produced from the manufacturer of the phone, we can’t always guarantee that your phone can be unlocked. You also need to be aware that we can’t guarantee the mobile phone features for use with another service provider this is sometimes because other providers have different network settings. If you have any further queries regarding this, please contact me on 08454122754 (calls to this number are charged at local rates if you’re a BT customer, but if you’re with another provider it may cost a bit more, so please check). Thank you for contacting me about unlocking your phone CUSTOMERS NAME. I’m pleased to have been able to give you your unlocking code and I’m sure you’ll enjoy using your phone. Yours sincerely NAME Customer Administration 8.30am – 4.30pm Monday to Friday