Hi Margaret,
By my calculations if you were to spread a 2″ layer on each bed you will need 65 ltrs per Bed. The depth you put on depends greatly on the current state of the soil, weeds etc.
As for the green manure, some people just chop it down and leave it on the surface but most dig it in. I personally don’t bother with these manures preferring instead to spread compost on the surface. I’m sure Charles will give more comprehensive advice on that for you.
If I could give you one piece of advice. As we are now coming into Winter I would suggest getting Charles’s Vegetable Course Book or his Vegetable Garden Diary. With the inevitable poor weather you will have lots of opportunities to read through these excellent Books. They will really help you to get started on the right path and ensure you are set up and ready for next spring.
Don.