As it's a T-mobile SIM, I went onto the T-mobile website to find out the PUK code (I know you can get the O2 ones off their website fairly easily) and indeed they have a page to tell you how to get your PUK code.
Key highlights:
Have you entered the wrong PIN code several times?
...
Step 1 Sign up or log in to My T-Mobile
...Not signed up for My T-Mobile yet?
Sign up for My T-Mobile now....Once you've signed up, simply follow steps one, two and three above to access your PUK.
So, I followed their instructions (yes, completely out of character for me) and signed up.
As with many sites, they send you an Activation PIN to clarify you are who you say you. Unsurprisingly, they send this to your mobile.
Erm...
So, I therefore need access to my mobile to get the code to access the code to allow me to access my mobile?
Erm...
Well, at least I got a good laugh out of it all...
(Rang T-mobile this morning, as my SIM is from a third-party, I need to get the PUK code off them and their phone lines aren't open on Sundays. Bother!)