Feeding purple sproting broccoli

I don't think it's worth feeding them with anything, 'til it warms up a bit.

Then I use Wilko's liquid veg food only a couple of £s a bottle, green veg need less Potash than tomatoes.
Alternatively put some chicken manure pellets around the roots, if it is dry, water it in well.

How  fortuitous, I was going to make a purple broccoli thread and there is one right here to add too!

I planted mine last year, they grew and then got mangled by caterpillars. They recovered beautifully and so far have overwintered but are now being mangled by pigeons.

As these plants are now almost 12 months old...when do they start producing?

If you start them off too early they make very large plants, which makes it difficult to net them. Start them in June and you get smaller plants but plenty of spears. If you have pigeon problems (and most of us have) you should net them. I use debris netting which is pretty good for keeping butterflies off as well (until they discover the holes!).