It really should be the start of the busy period of sowing if the weather has been kindly in March and allowed you to get all the preparation done. But despite April being the go for it month there is always the word of caution – in the North East we know that chilly nights still abound and that the ground may not be as warm as in more southerly parts of the UK. One of your best guides is the appearance of annual weeds – once they begin to thrive the soil is really warm enough to plant.
Anyway – advice from The National Allotment Society and from the RHS and from a well known allotment site. A good source of advice is often written on the seed packet of the crop that you want to grow. How many times will an experienced gardener ask you if that was written on the seed packet!