Hi I planted purple sprouting broccoli in February ( probably not the right time) but the plant have come on very well maybe too well some have heads on them now but they are very loose not clustered together like green broccoli. Some also look like they are about to flower. Not sure if it has been a waste this year. But never having seen purple broccoli not sure if it should look like the green.
The flowers are much less compact than green broccoli and the stems are more tender so do pick the heads now before they open and turn yellow. In future, pick them younger. They'll keep on cropping.
Mine have just produced their first edible heads which is earlier than usual but I planted mine earlier on purpose so I'd get crops before they get frozen to death in winter. Haven't managed to over winter any winter brassicas for the last 6 years it's been so cold here.
Purple sprouting broccoli doesn't form large single heads but lots of small ones. They are long and thin. It sounds like yours are ready to start harvesting - just cut each head off before it flowers and more will form - it will probably continue providing heads for many weeks as long as you don't let it flower.