Purple sprouting broccoli heads rotting

[ameno]

Some of the heads of my purple sprouting broccoli are rotting on the plant before they are ready. The buds just turn to mush. It shouldn't be cold damage, as it has only been down to -3 at the coldest here, and it ought to be able to withstand well below that.
Anyone know what is causing this and how, if possible, I can prevent it?

[nickdub]

Definitely won't be the cold. Could be some sort of physical damage first like pigeons pecking it, which is then letting the rot in.

[burnie]

Have you grown them in the same place in the last 3 years, are they well spaced to allow air flow? I don't think you can save them if it's fungal/bacterial.

[ameno]

They are in a new area, and have plenty of space. In fact, most often it's the terminal head which rots, which would obviously have the best airflow of all of them.
I don't think it's pigeon damage letting in rot, either, because there are no signs on any of them of pecking. I should cover them soon, mind, as last year the pigeons started their attack in about mid February, so it'll probably be the same this year.

So far, I've just been cutting out the affected heads and disposing of them to limit any spread. It's not affecting all of the plants, and nor is it affecting all of the heads even on affected plants. Only the head itself rots. Once the rot reaches the thicker stem, it just stops. It's probably bacterial rather than fungal, as there are no signs of fungal growth.

[Penellype]

I find brassica heads sometimes rot and I reckon it is due to excessive wet. There has been a lot of rain/snow recently so that would be the prime suspect.