Broccoli really need to be grown in the open ground. Added to which this has been a really odd growing year! A lot of brassicas have flowered in the same year they were sown - even though they are meant to me biennials. Sorry to say your broccoli will not now produce edible stems.

The broccoli that we eat is actually a cluster of flower buds. If it isn't picked the flowers will open, which is what yours have done. Once the head appears to be fully formed you might as well pick and eat it even though it is smaller than you were expecting.

Our broccoli only ever produces small heads...

thanks for your comments and time to reply back to me thats just answered what i thought .

Just keep on growing and accept that you wont get the monster heads that the supermarkets sell - god knows what they do to create heads that size!