Ok, a follow up to my post yesterday. When I said it works a treat I meant the script. The followers I got were quite disappointing, basically 10% followed back.
However I think this is due to the quality of the list, there are quite a few blocked accounts in the 100 I tried, which were not the first 100 as I randomised them.
There are quite a few good follow back lists on twitter that I have used in the past to build an untargetted following quite fast so I think by using a script to download lists of followers and uuse them in the OP's script is the way forward.
I just need a script to download followers.