I never use slug pellets because I find they increase the problem instead of helping. I have found Murphys Slugit, a liquid does work. It is not available everywhere but B&Q and some garden centres stock it. If you can reach them pick of the offenders and drop them in a plastic bottle with some bleach in it. That will kill them although it sounds cruel. Then put the whole thing in the bin.

Have you tried slug/snail copper tape it will stick to most things this stuff does work I tried some around some empty tubs to experiment with the trails went as far as the tape and then to back to the ground