Just watch their poo - if it looks runny or a funny colour then ask vet as could be coccidiosis it is a bug that occurs naturally but animals can pick it up and if not 100% themselves can get sick from it. They should be fine on that plot as they quickly destroy whatever they are on anyway! They will also need worming every so often - they have come out with a spot on wormer for chooks now which is 0.8% ivermectin that you drop on the back of the neck so alot easier than stuff you stick in feed! You also may need to stick in a tray with sand in it so they can have a dust bath. I love my chooks - they are so funny to watch!
You can either buy it on ebay - type in Ivermectin wormer chicken and it should come up, it was about £26 for a small bottle to do 30 large hens - or the cheaper option, which is what I did, was to go to the vet and get them to dilute the cattle Ivermectin spot on, which is exactly the same stuff just stronger, down to 0.8% and it is 4 or 5 drops on the skin of each bird - obviously less for a bantam. It does all the creepy crawlies as well as the internal worms.