Hi Hugh,
A layer of compost 4-6″ deep is all that is required. However for a large area such as yours that’s a lot of compost to try and get hold of. You can do just as good a job by putting a layer of well saturated cardboard down first and covering this with a 3″ layer of compost. That will still give you a reasonable depth to sow a variety of veggies into immediately and it certainly will be ready for you Garlic in October.
I would strongly advise sowing something in the area now rather than leaving bare soil until Autumn.
Don.