It's probably slugs/snails. Either use slug pellets or if you're more environmentally friendly, grit around the base of the plants, crushed egg shells, coffee grounds all seem to work reasonably well.

Suttons seeds also sell sheep wool pellets which will rot in to the ground.

http://www.suttons.co.uk/Gardening/Garden+Equipment/Garden+Pest+Control/Slug+Gone+Pellets_584360.htm#584360

100% natural pellets
A safe, NATURAL WAY OF GETTING RID OF SLUGS AND SNAILS without killing them .... AND AN ORGANIC SOIL CONDITIONER TOO!
These 100% pure sheep wool pellets are a chemical-free, environmentally friendly way to protect flowers, fruit and vegetables from slugs and snails. Simply scatter around plants in beds, pots or baskets and water in, and they form a barrier that slugs cannot cross. 10 litre pack (enought to protect 60 young plants). And not only that - they also:
Act as a natural, long-lasting, weed-suppressing mulch
Absorb and retain moisture
Biodegrade over 6-14 months, releasing organic nutrients
Improve the condition of your soil

The same thing happened to my Salvia plants (whole plants were eaten so that only the stalks were left). I put out the crushed eggshells and coffee granules which worked really well for every other plant except the Salvia! A friend suggested it was mice but it was too late for me to do anything about it, or even test the theory, as by that time the Salvia were all gone! I hope you find the culprit!

I've given up on marigolds for the same reason - weeks of TLC just to make a slugs breakfast...

Thanks every one

I'm with Alex on this one..Slug gone is very good,and widely available at garden centres now..but it's best dug into the soil as early as February,before the eggs hatch.it catches the little B's before they get chance to breed...It is also acts as a fertiliser,but scattering on the soil. now,will still help...I used to buy it from a local Nursery,,before it changed it's name.

Mizzle, I have the same problems with Salvia. I have replaced them 3 times, and have given up.

The slugs and snails keep eating them, no matter what.

Bloomer, I recommended it as I have used it myself, just smells a bit like sheep (obviously).

I thought you must have,Alex..and when I first started using it,it was called Sheep....it.:o) That isn't me bleeping anything out,that was the spelling on the packet...You can see why they would change it to slug gone,for sale to the public..Lol.

Who needs chemical slug controls when you can use natural deterrents or slug traps?

Alex and Bloomer, is Slug Gone a safe thing to use in a garden that's part of a hedgehog's domain? It sounds good but I don't want my hedgehog to become a fallen victim of my war on slugs!

Slug Gone is a natural product made from sheep wool.

The Slug Gone is not toxic, the slugs just hate the texture of the wool pellets, so they give the pelleted areas a miss.

"A safe, NATURAL WAY OF GETTING RID OF SLUGS AND SNAILS without killing them .... AND AN ORGANIC SOIL CONDITIONER TOO! "

"These 100% pure sheep wool pellets are a chemical-free, environmentally friendly way to protect flowers, fruit and vegetables from slugs and snails."

The hedgehogs will not be harmed by the use of these wool pellets.

oh I see, they don't actually eat Slug Off! Excellent, I think I'll give them a try at the weekend. Some of my plants are beyond saving but the others have a chance!