hi
It's a nice sunny day, so I let the large hermann's out into the outside pen, after about five minutes I heard a crunching sound, it was one of the tortoise's eating a snail ,is this normal.

Yes it's normal - they eat the shells for calcium. Having said that they also eat slugs sometimes. It can leave a horrible mess on their mouths and beaks though. Cold black tea is good for removing this goo
Sue