How much sunlight does the plot get? This can be a big factor for a lot of crops.
Soil - how quickly does it dry out? Some of my potato and garlic yields this year was well down because it got hot and dry and I underestimated water needs at a time they needed it to bulk up. The Leylandi hedge could be taking a lot more moisture out of the ground than you realise. Why not try trench composting for the runner/french beans (basically dig a trench about now and throughout winter add kitchen waste such as tea bags, fruit & veg peelings except potato etc, then in late winter/early spring cover the trench with soil and leave it until you are ready to sow/plant beans in late spring) Beans can be thirsty plants and the compost will help to retain moisture during the hot weather. When watering during hot weather, give the ground a thorough drenching (push your finger into the soil to see just how far the water has gone - even when the bed looks saturated and puddled after watering, if it has got too dry you'll be surprised to find that you'll need a lot more water than you realised) Onions need water to bulk up and if I remember, it was hot and dry in July when they needed it the most.
Soil nutrients - maybe you need to add more than manure. High nitrogen levels can give leafy growth at the expense of root or fruit crops so maybe you need to boost potassium and phosphorous levels, and then there are micro nutrients such as magnesium, manganese, calcium, iron etc needed for plant health. You say you feed once a week - with what? Different fertilisers have subtle variations in nutrients for different purposes (high nitrogen for leafy growth, high potash for fruit and flowers) Maybe rake in some general purpose fertiliser into the soil a couple of weeks before sowing/planting - look for a brand that contains the micro nutrients in case something is lacking. With regards to manure - the nutrients are not instantly released into the soil but needs to break down over time, so it might explain why some things did better this year than last year.
Soil pH - that Leylandi is troubling me and makes me wonder if it could be affecting the plot. Soil pH affects the uptake of nutrients. If the soil is too acidic, then some crops like brassicas and beans don't do so well. Maybe you need to add some lime to lower the pH (you can put a handful into the planting holes of brassicas) Don't forget to heel in brassicas as they like firm soil round their roots.
Weather - is actually the biggest factor of all
Last year was very wet so going back to the other points, rain could have leached out nutrients, not enough sun, affect soil pH and some crops did well, others not. This year a very late cold spring (so the ground is cold and everything is slow to take off) quickly followed by hot dry weather (some things don't like hot weather, others do well) and plants react differently so it is difficult to compare one year's crop with the next - I think you'll find everyone will tell you that a particular crop did better or worse than expected and every year it is different. I also found the temperatures see-sawed a bit this year - a hot spell, followed by a cold snap ... some plants might have got checked.
Numbers and spacing - if you planted too close, then they could be struggling for light, water and nutrients. Likewise, keeping weeds under control is important for the same reason (onions in particular can get stunted if crowded) Numbers - if you don't grow that many, then harvests might look dismal. And then there are expectations - try not to compare your harvest with supermarket produce which is graded. Also, genetic variation is a factor too - I can grow a row of say carrots, and some are humongous, some are tidgy all in the same row with the same growing conditions but overall the harvest would be fine. If there were only a few, the harvest might look pathetic if you understand what I mean.
Crops - some do better than others in any given plot. When I had 2 plots, they seem to be good for different crops. Caulis are supposedly not a beginners crop so take heart if they didn't do well. Leeks - they might fatten up a bit yet, or you might need to look at a different variety? Did you dib a hole and pop them in, or were they just planted with the stems above ground?
Sorry it is a long post but there could have been any number of reasons why things have not done well. Look on it as a learning experience rather than a failure and don't give up yet!