Hi Roger and welcome to GoY. To start with the area you are talking about does not really get enough sun. Veggie crops grow best in full sun.

To start with I would suggest you get hold of a few polystyrene salmon fish boxes, leave them out in the air for a few weeks to get rid of the smell. Paint the outside up with masonry paint and make some holes in the bottom fill with good humus rich compost and grow your herbs, salad leaves,spring onions garlic and possibly carrots in. Carrots, and parsnips, require a very fine tilth and a light sandy soil to do well. Parsnips need a good depth of soil to do well. Leeks are not really going to do well in fish boxes nor, unless they are very deep raised beds. Runner beans require a well composted or manured trench to sow the beans in for best results but you could try french or dwarf beans.

The reason I am suggesting you start off with the fish boxes is it will give you a chance to see how well things grow without too much cost. If you decide to create raised beds on your decking a) what is under the decking and b) how are you going to keep the soil in the raised bed/ stop the water from going everywhere.

Many thanks. The polystyrene boxes are a great idea as an inexpensive trial and will give that a shot this year. Thanks also for the specific advice for the different crops.

You're welcome