Steve
I dont know where you are in the UK, but in S&E England it has become a nighmare of a weed. It rarely produces seed, but it propogates itself from the smallest amount of root. It has been spead by land cultivation and earthmoving projects, and apears to have no significant pests. Planting it in a container is a really good idea. The plant grows like a dockleave with thicker and more crinkly leaves. It is very common but it blends into the landscape. If your intention is to make horseradish sauce then be careful. Grating the roots will make your eyes sting very badly.
MacD