Your cloche will give them warmer days, and keep the wind off them (which is as important - even a light breeze will dessicate the leaves of the young plants, and if they are not growing vigorously, as it is early in the year, the breeze adds additional stress. So cloche is good 
On a cold night chuck a blanket, or old duvet or similar, over the cloche

On a cold night chuck a blanket, or old duvet or similar, over the cloche

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