was just about to mention badger .. but you beat me to it lol !!
give them time, lots of handling, get them used to you, your voice, being on floor while you step around in big boots, your dogs, tunnels, get them used to everything they'll see on the day, i also take me ferrets out every now and again to some land, just to get them used to being in a strange place with me and me dogs, then its not such a surprise for them, also the bigger the run they have in your garden the fitter they are and longer they can work
best of luck, but time and patience is the key
hope this helps