Winter and autumn Brassicas

You can grow them anywhere that didn't grow brassicas already this year 

Timing would be difficult if you tried to plant them after French beans.... I'm still picking my beans and my cabbages were planted quite a few weeks ago.

I try to grow mine where the broad beans were. The nitrogen in the nodules of the bean roots are good for brassicas. 

According to recent research this is not true as although the beans do fix nitrogen from the soil, they then use it up when they set their seeds, so very little is left in the ground. Makes them good as green manure if you dig them in before setting seed.