I have a photograph here actually, taken not too far from Brynmair, a sheep gathering and dipping, probably around the start of the last century, as you say, even back then it was mandatory and you had the local police present to ensure it was done properly
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I was watching a farming programme on S4C a few weeks ago - following some lads just over the mountain who now have a mobile sheep dip and dipping service, so no need even any more to have a ffald and sheep dip on your farm, they just roll up on to your farmyard and the sheep are just guided from the stockyard up onto the trailer and off the other side.