[JayG]

Please elaborate on this trench because I tried the same thing last year, dug out a trench and filled it up with waste materials but when the time came to start work on it the waste material had shrunk to nothing so I was left with a deepish trench that I tried to level up with the neighbouring soil. Everything is fine so far. Nice runner beans, no infection  but what is the proper way to go about this, please?

Your waste material can't have literally shrunk to absolutely nothing, although obviously it will have reduced in volume as it rots down - the soil you excavated to make the trench should more than compensate for that when you cover the trench to plant your beans in.

The reason many people (including me) get away with growing beans in the same bed every year is that there are very few persistent soil-borne diseases they are subject to (the process of digging a new trench, filling it with organic material, and back-filling with soil, preferably from an adjacent area, is nearly as good as rotation anyway.) 

[mumofstig]

Dig a trench about a spade width and deep, line it with  a thick layer of paper then add your veg peeling, apple cores, cabbage stems torn cardboard - anything that will rot and cover with a thin layer of soil to deter foxes, then add another layer etc etc until it's full. Mound up the rest of the earth you removed over the trench, and mark each end with a cane.
It will sink level as the compost materials rot down, but shouldn't be lower than it was because you have put back as much earth as you dug out.