why have my onions got a flower growing on it?someone said its bolted??? please help

often they do this when they get over dry or stressed. flowering and setting seed is the plants survival strategy.
i would nip the flowers off and keep them watered. some one else may have better advice for you.
welcome to GoY too by the way.

thank you seaburngirl for the welcome and advice

Yes, quite a few of my onions have bolted this year too. Seaburngirl is quite right - it's caused by stress, such as being too dry. We had that period of hot dry weather with a constant breeze that dried everything out if you remember. I'd yank out the ones that have bolted and consign them to the compost heap - that will at least benefit the remaining ones. Keep them well watered in the future.