Sprinkle slug pellets around the base of the plant.
I know some people don't like using them but all I can say is ....it works for me. My only problem is remembering to do it before the slugs get there.
And welcome back Jonathan.

I do that, but it doesn't seem to kill them all. It just reduces the population. I use ferric ones.

There is a product made from sheep wool called slug gone, I haven't tried it but supposed to work
The other thing I heard was someone cutting up those copper pan scourers and spreading them around susceptible plants, slugs don,t like copper

I should have said I have copper tape around the pots. But they somehow seem to get across, which is a bit of a mystery to me. Is there anything that can be fed to the plant itself to change its taste so as to make them off-putting? It is the only solution I can think of. Would anyone happen to know how?

Apologies, my eyesight is going and will not return until tomorrow.

Copper tape does work, but it has to be thick enough for slugs and snail not to leap over - say at least an inch. They also attach to plants via low leaves or from other plants close growing close to it. Clever things.

For what it's worth I use a product called Slug & Snail Killer by Bayer Garden. I get it in 1kg bottles. The main thing is that I pay no attention to the instructions about how much to apply. I whack it on until I can't see how anything could get through to the plants. It's amazing how many snail shells I can see afterwards. The product appears to make the body of the slug or snail actually decompose, so there's little sign of the slugs.

Per Kildermorie's answer, you do really need to look under leaves. In fact, look anywhere that isn't immediately obvious. They really are sneaky.

They were talking on GQT radio 4 about using nematodes....
I haven,t tried it but think it attacks the slug

Surrounding a vulnerable plant with coffee dregs is supposed to work as well

http://www.growsonyou.com/2ndhand/blog/20128-slug-and-snail-deterrents

I have looked up them 'Bayer' one, on amazon. But there is no indication of how it works. Does anyone know?

You'd have to be a prolific coffee drinker Andrew. And you'll be buzzing from caffeine. ... I did try nematodes once but found it expensive to keep up.

The slug gone is made from sheep fleece, it seems that slugs don,t like the lanolin then the fleece breaks down in the soil.....no idea if it works though

Someone I used know had a garden full of Hostas and of course I asked about keeping snails at bay. What he had done was put all his Hostas in large pots and stood them
on a area of peashingle large enough so the snails couldn't traverse. Any good Jonah13?

You should see how tiny my garden is Jcee. Not enough room for a square foot of pea shingle. Maybe hort grit would slow them more?

Crushed egg shells seem to work Jonathan, they hate trying to slide over the sharp edges.....we also have tried sinking a jar half full of beer in the garden, and this did get rid of quite a few. Our garden is absolute snail and slug heaven, when it's dusk, I go out and collect as many as I can by hand and dispose of them.