if the ditch is up top the field its fine to put the spoil on the side of the ditch, if its down bottom then it can be a bit silly depending on the lie of the land, then it wants carting away to fill up odd hollows or bits that don't have much topsoil, this is the obvious thing to dobeing serious, I am pretty sure, one reason there is no dredging is because the "spoil" is classed as contaminated waste and has to be disposed of as such, and I think if we did a chemical analysis of it, it would be too, I am not sure if it is cross compliance or a glastir reg, that you must not use spoil from a ditch to level up low spots in a field (I can't for the life of me think why not), if you were to always spread next to the ditch, you end up blocking surface water running into it!