What is nibbling my courgette leaves?

I've got about 7 plants that have come up beautifully in my plastic greenhouse with 2 leaves of about an inch and a half each in size yet two of them are having one of their leaves attacked by something - one to the point it's vanished.  I say nibbling, however they start to look a bit more shrivelled than anything so I don't know if it's an insect or some sort of disease?  Has anyone else experienced anything like this, and if so, can anyone provide any advice?  If it's slugs they must be invisible ones 

Now there's a question?  I don't know but the greenhouse steams up pretty quick when it's zipped shut so maybe too hot.  Then again, it's not exactly an Indian summer is it and once or twice I've left it open carelessly overnight..  who knows?  I don't see slugs getting to the top shelf of my placcy greenhouse so I think I can rule that out!

Now there's a question?  I don't know but the greenhouse steams up pretty quick when it's zipped shut so maybe too hot.  Then again, it's not exactly an Indian summer is it and once or twice I've left it open carelessly overnight..  who knows?  I don't see slugs getting to the top shelf of my placcy greenhouse so I think I can rule that out!

I was amazed to find a couple of snails on the 4th floor of mine havin' a go at my sunflower seedlings!   

Sounds like it might be the slugs after all then.  Should I put salt around the stem?

Sounds like it might be the slugs after all then.  Should I put salt around the stem?

Not sure salt is the way to go .... I've used pellets, if your against that then I just read on my can of wd40 that you can spray (with care) around the pot base and they cant climb up! or they dont like the smell of coffee grounds so you can sprinkle that around.

However, I left my gh window open overnight and a snail has climbed in through the window and down the window ... I think if i hadnt of came in it would of climbed onto my plants from above!!