Broccoli help needed - huge leaves, no broccoli??

I'm sure it's that i've done it wrong again, but I bought some broccoli plugs from the garden centre, planted them, they have been doing great, or so I thought, but only one of them has formed anything like a broccoli spear, but they were pathetically tiny ones, I thought that would be fine - they would just get denser as time went on, but today one has bolted, so that's the end of that.

There are 6 plants in total, I planted them the way the instructions said to (though experience in the past tells me these can be wrong!) they were planted about 30cm apart, in rows about 30cm apart. The two rows are between two rows of potatoes and two rows of sugar snap peas, so in hindsight, I haven't given them the best setup really as you have 6 rows of quite big plants, all wanting lots of light

5 out of 6 of the plants are growing absolutely huge leaves, but not a sign of any kind of broccoli spears, the one that is growing broccoli spears, but has bolted, has smaller leaves.

I did think that maybe it was too early for them to be sprouting, and the one that has is doing it because it's in the middle of the bed, in the middle of other plants so has the least light, so is desperately trying to win by bolting? I don't know when broccoli is meant to start sprouting

Any help appreciated!