Celery

I have noticed one of my 6 celery plants has started to bolt :( but the stems are still very small, should I pick it and make do or leave it?
This is the first time iv tried growing celery and thought it was doing very well, it was sowed from seed in march. Would i get away with sowing more seed or have i missed the boat a little?

 

I bought some plugs off ebay this time last year and they did very well. I think that would be your best bet.