Thanks. I've tried those iron based pellets but they weren't that effective and dissolved in the rain quite quickly.
My preferred option is to use copper rings. It's the only alternative to pellets that actually works. I should have put them round the baby cabbage plants this year, not sure why I didn't.
I started out years ago by buying various types of Cu tape and mesh and other purpose made products. They were a bit rubbish really; didn't work that well and didn't last very long. I realised that I could do far better myself starting from standard copper plumbing pipe and some basic metal working.
I cut 22mm pipe to a length of about 12", slit it along its length, bend and beat it out flat to a ribbon and then roll that around the other way to end up with a ring of 4" diameter and about 2.5" high. I put these around individual plants in the ground, or I use them to stand pots on if I'm protecting plants in pots. (This should make a bit more sense with the photos).
I've been using them for a number of years now and really happy with them. They're not 100% effective but getting there. Nothing is against slugs to my knowledge. I'm not sure of the exact chemistry but for some reason but slugs hate copper. I've tested it and watched them visibly recoil from it!
It takes a fair bit of effort and time to initially make the rings but I figure they'll last forever now. Being copper they won't rust away obviously. Also pipe is quite a thick guage they are strong enough to sit really heavy pots on.
If I was a salesman I'd have been on Dragon's Den with the idea before now! Never seen anything as good available to buy.