Thanks for the advice everyone - technical and legal - sorry for late reply!
It was just for internal purposes for a bit of fun for the christmas party (stitching people up basically!) evil but potential for a lot of fun! we wanted to ask for anonymous stuff but really its not anonymous.
Some good stuff to look at there though so will have a look through it for future use.
For the record - we buy opt in databases where we already have email addresses of people but its a good point - it didn't occur to me my post would come across like that!