The one at the Glencorrig centre is great, with the 'open until you leave' cafe/bar just next door - and you can ride all the trails without driving anywhere.

Why stay anywhere else?

They've had this problem before, and last time we were there they had a onsite security guard - give them a call to check?

Also whenever camping we secure our bikes to an immoveable object, usually backing the car cw rack onto a large tree and using a secure-wire plus locks through all the major components.