I use an off the shelf rat poison, which you can get at Wilko, B&Q etc. Either put the bait in the middle of a length of drainpipe, laid on the ground and supported by bricks so it doesn't roll.
If you've got some spare bricks, build a little shelter just 1 brick high and totally covered with bricks, with just a 7cm gap in one of the sides, and the bait inside.
Normally works within a couple of days.