Update:
Terriermen couldnt come this weekend, which turned out pretty well actually. It turns out that the poison did indeed work, it just took longer than the shop told me it would.
I had a couple of guys staying overnight in the yard on Friday, and they saw no rats at all, but did find two corpses. Bearing in mind that these rats used to march right past in single file in broad day light with one middle claw raised at me not 2 weeks ago, this was great news!
And yep, they were right. The great rat massacre of 2011 has truly taken place. I spent yesterday morning locating and burning bodies.

I dug 12 big well fed adults out of one straw bale alone. I know there are at least 4 other colonies, but they're all underground, so they can stay there. I wonder how many I had in total if I have physically disposed of about 25 bodies? Lots anyway..
I am now resolved that they will never get out of control like again, so I never have to use poison again.I know it had to be done, but I still hate it.
Wish me luck...
Thanks people.