If you sowed your broad beans in pots earlier in the year, they will be ready for planting out in April (maybe the last week in March if the weather is good!).
The generally accepted advice is plant them in double rows allowing 20cm-25cm between plants and 30cm or so between the double rows to allow access. If planting in a raised bed where you don’t need to walk between rows, then you might consider diagonal rows 20cm apart and 20cm between plants. (There is method in this approach which I will explain later).
Broad beans are amongst the most resilient of legumes and will tolerate low temperatures, but you will need to cover them with fleece if frosts are still likely. (I’ve got a micromesh frame that fits over the raised bed with a removable top cover which also protects my plants from other hazards such as pigeons in the early stages of growth).
Depending on the variety, the plants grow to a height of around 1.2m and will need some form of support. But unlike peas and runner beans, broad bean plants aren’t climbers so using cane ‘wigwams’ or a climbing net won’t work.
Using posts at the corners of the bed and enclosing the plants in a string framework will do the trick although a better method of supporting the growing plants is to use canes and then form a lattice work with string at different heights between the canes. Here’s where diagonal rows can help. Using canes around the bed and spaced along the diagonals, a criss-cross string lattice is easy to build allowing the plants to grow up through the ‘cells’ of the lattice.