You're worrying too much. Stop worrying too much.
Are you going to force every potential employee to sign an NDA because you're afraid they'll tell others? Are you going to avoid explaining to people at meetups what your company does? Etc. etc.
What about the chance that a big company is going to copy what you do after you launch? All startups must be screwed then (sarcasm). Etc. etc.
I should have mentioned that in my original post. I do tend to talk about my idea with everyone. It helps to weed out poor ideas and germinate new ones.
My question really was literally to do with a direct competitor getting an insight into my business plan while I was in the middle of building the MVP.
I think you are right in that i should stop worrying though :)
Also: I got a message from another mate in YC. He said that it's mostly the guys that are close with YC that do a lot of the screening so less worrying ahoy!