Is it 4 inches or 3 inches maximum Dave?

I believe one can possess a knife here as long as it can be justified for one's work. My neighbour in the last village was a carpet fitter, and he had a set of ferocious-looking knives and cutting tools on his van. He was allowed to have them, but not to select one and carry it about on his person.

Actually, even a 3 inch blade could kill someone if it was extremely sharp.