April: 

Potatoes:  Plant Second earlies at the start of the month, be aware of Frosts

Beetroot: Beetroot can be sow in modules and then planted out

BroadBeans: If sown in pots; these can be planted out now. 

Carrots: Rember Carrot root fly and make sure you protect your plants

Chard

Cauilflower (Summer type)

Kohlrabi

Leeks

Lettuces

Radishes

Turnips

Onions (plant out Shallots)

Peas

Salisfy

Scorzonera

Spinich

Tomatoes  (If you have a greenhouse these would be from seed)