Fennel bolted?

My fennel smhas been growing fast but not notices what looks like flowers coming. Haven't grown fennel before so any advice greatly recieved 

Are you growing herb fennel, or Florence fennel, the one which forms a swollen stem to eat?
Your pic looks like herb fennel, which does need to flower to set seeds to save.

If you planted Florence fennel, then the flower isn't good news. Perhaps you sowed them too early? April or May sowings are probably ready for eating about now. So if any have formed bulbs eat them up asap.
IMO they are best sown in June and harvested in September, they are less likely to bolt in cooler weather.

It looks like herb fennel. Which is lovely but won't bulb up.  Otherwise it is bulb fennel but sown too early - it also needs regular watering.

I sowed mine about 3 weeks ago and it's just getting going.   

After several years attempting to grow Florence Fennel which I could actually eat () I came the closest yet last year, but sowing it in mid July proved a bit too late for it to be fully developed before the cold weather stopped it in its tracks. Did manage to harvest some small 'bulbs' which at last were tender and tasted good.

Meant to sow it in early July this year but forgot, so have probably made the same mistake again - key things are an open sunny position, well drained but fertile soil, and plenty of water.