Chicken Colditz!
Lots of ideas combined together to keep the little darlings out. you want to make the garden unattractive to predators and your girls really hard work to get at. easy to say...
L shaped aviary mesh buried under the ground, preferably with hard floor (paving slabs, gravel etc) to discourage little blighters digging under the wire. Chicken wire is too weak (even a bunny can snip through it - I found out the hard way).
If you can get a current going through it, so much the better.
Keep spilt food to a minimum (try feeding on a tray so spilt stuff can be swept up) and try and mask the smell of your chooks with garlic and other smelly plants. Or stray teenager. (Will repel anything, even biological parents if unwashed for long enough.) Try disturbing the ground at the edge of the garden. it might make them nervous, but you will be leaving human smells behind - and we are top predators!
An ideal is to keep the chooks somewhere close to the house, and solid like a garage, preferably used by people. it won't stop them, but they'll think twice. (I'm the saddo who wired a radio and light under the shed to discourage foxes from making a den)
Maybe attracting wildlife will create a means of polishing off unwanted chicken food and provide an alternative food source for the predators. Best done in long term with careful planting of bushes hedges etc. Failing that a fruit bush will keep the voles and birds full. Just don't go thinking you'll get some as well - those animals are professionals! Other plants such as gorse might make the entry into your garden less attractive. Be creative, or ask the RHS since they live for this stuff.
A fully armed SS-trained Alsatian with a rapidly reloading machine gun would also help, but may contravene council rules.
Good Luck!