I personally think there is already enough paperwork involved in day-to-day life without adding to it when taking the ferrets out.
But – the time a letter of written permission really can help is if you get pulled by the Old Bill when you’re working a farm.
You can then prove you’re not poaching, which is nice, and you don’t have to down tools and disturb the farmer by knocking on his door at inconvenient times of the day, or night.
What I do recommend is that it helps to phone the police if you are going to use a lurcher.
Here in Hampshire the simple sight of a running dog can cause such hassle from some officers, that you would believe they’re totally ‘anti’ us ferreting types!