[MyAchingBack]

I have just created a seed bed on my allotment. Is it advisable to grow the seeds under a cloche or only when a frost is due?

[DD.]

Depends on the seed. One rule does not fit all.

What are they?

[MyAchingBack]

I was going to plant brassica and leek seeds.

[DD.]

Neither would really harm from a bit of protection.

My preference is in seed trays or modules to get better germination, even if it's outside at home in a sheltered spot.

[steved]

Id cover it if i were you. Its unseasonably mild at the moment but it wont take much bad weather to drop the soil temperature to a point where seeds will struggle to germinate/thrive.
Of course, it also depends what youre gonna sow in it, brassicas,leeks etc should be okay but i would be careful with tender crops just yet.