Spring cabbage confusion

I have grown Durham Early variety over winter for the last few seasons, I always wait for them to form hearts before harvesting, but I recently read that true spring cabbage are just the leaves. They do eventually form hearts but have I been waiting too long then? Do any early varieties form hearts in the spring at all? Thanks for any info, I have looked on line, but there are many different answers to the same question, hence the confusion.....well, for me anyway !

We usually grow "winter green" . You can take leaves off as "cut & come again" or it hearts eventually if you leave it alone. We leave them in the ground after taking the heart off & they make 3 or 4 new small hearts from the stump. We're still eating "Spring" cabbage now & will be until autumn! 
https://tamarorganics.co.uk/product-category/vegetable-varieties/cabbage-spring

I have to say we have the same problem with our spring cabbage and although we raised the seed in the poly tunnel and then potted them on to overwinter before spring planting in the allotments we ended up with just leaves

having tried several varieties I'm giving wheelers imperial and spring hero a go this time then if they don't heart then maybe its better to go with an early cabbage like caraflex F1 sowings from January?