Question
Is there any way I can prevent maggots attacking my turnips?

Answer
Dear Helene,
Since I am unsure as to whether the turnips already harvested or still in the ground, I guess I can just address both.  If they are harvested, you need a cooler place to keep them.  Maggots are baby flies, so if it's too cold for flies, there will be no maggots.  And turnips will be fine just above freezing.  If they are still in the ground, perhaps a bit more acid environment will be a deterrent, but go lightly and cautiously when changing the pH.
I wish you well.  
Kind Regards,
Kindred
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