Does it generate a new head? Or is it then done for the year? What can I do to encourage larger heads?
If you cut off the head (central spear) when the flower shoots are well developed, but before the tiny flower buds have opened, the plant will not grow another one, but will produce side shoots which you can pick regularly for five or six weeks; however, if you let any of them flower, the plant will stop producing them.
If you choose a sunny site, work enough manure or compost into the soil in autumn and, if necessary, lime (the ideal pH for broccoli is 7.0 - neutral) - better to manure and/or fertilize first, and then wait at least a couple of months before applying the lime - you should get some good-sized heads. Corvet is a very good variety: it produces a large head and, once decapitated, a succession of secondary shoots.