LAND on it's own does not have any council tax. It is only the building that may attract council tax. I'm not sure if a polytunnel qualifies as a building
However, agricultural land is defined as land used to grow food for human consumption (eg Fruit and veg) or fodder for animal consumption which are intended for human comsumption (eg grazing land for sheep and cattle). You do not need any permission to change from grassland to vegetable production. However, putting a horse on grazing land would need change of use (unless you use the horse for agricultural purposes...eg ploughing, OR intend to eat the horse!) Some areas are very strict about this (spoil sports).
I would suggest you write to the council and get a written reply so that they cannot argue at a later date.