I agree with Sneezi. Compost lots of lawn grass preferably from your own lawn -- or you can grow perennial white clover instead of lawn grass. Get the green manure and cover crop book from Johnny's Selected Seeds. It is concise and inexpensive. Fireplace ashes are terrific but don't overdo it; it is high in potassium; but you mostly want fall leaves for minerals and green stuff like grass for nitrogen. Also good, in moderation, is seaweed. Too much is dangerous because it has too much salt.
Buckwheat is a good cover crop and green manure. And snow peas or snap peas are a good first-time crop to start very very early. But for snap peas, you absolutely must get the climbing kind, not the bush kind.
I have to disagree with Arlyn about using a food processor. Unless your garden is no bigger than about16 square feet, the amount of material you can grind up in a food processor is insignificant. Plus anything small enough to fit in a food processor, is small enough to degrade rapidly without first being ground up. If you want, get a gas-powerd leaf shredder and log grinder for wood chips and to make leaf composting go a bit faster.
For fruit you can start with melons. Ask you university expert (I don't know what they are called in England) for the right variety to grow in your area. Choosing a good cultivar, or the right cultivar, can have more of an effect on crop yield, and quality, than anything else you do, except adding lots of organic matter to the soil, and making sure your plants have the right amount of water. (Some plants are sensitive to too much water; melons will lose their sweetness and have more leaf fungi; tomatoes will split; others, like sweet corn, seem to taste juicy if they have more water than might seem necessary.
I disagree about man-made fertilization doing more harm than good. If you depend mostly on industrially-produced commercial nitrogen sources, yes, you are doing more harm than good -- esp to the ground water and surrounding rivers and streams. But if you depend mostly on compost, and use a small amount of industrially produced nitrogen to "top off" your soil with the right amount of nitrogen for maximum yield, you are getting the hightest yield per acre, and not doing any harm. The large amount of organic matter in the soil assures that less of the industrial N runs off and more of it get taken up by your plants.
I also disagree about chicken manure; it adds a subtle chicken-feces flavor to many vegetables. Cow manure adds less of a flavor. Also, both cow and chicken feces can be vectors for pathogens. Microorganism species that grow on or in other animals, also grow on humans; microorganism species that grows on or in plants, do not. While compost made in an animals gastro-intestinal tract has a greater amount of N than compost made in a wooden bin, due to the faster-acting decay bacteria than that found in a wooden bin, the chicken also "robs" some of the plant nutrients from the compost, to use to make its flesh.
Before using rabbit feces, they have to be properly aged, to make them safe. There is a way that is said to make it safe, but I am not sure how this is done.
