Turned out they were from the *council (they were contractors so didn't have uniform) and were under instructions to fix people's fences on our street because they were falling apart. That is true, they are. My mum said that that was okay, but they should have knocked on the door and told us first. That's the big problem. The fact that they just walked into our garden and started messing around, without uniforms on. They could have been anybody. Also, they weren't even doing anything after awhile. They were just stood in our garden talking.
*The council is a local government. They rent out houses called council houses and provide service to the houses, sometimes by actual uniformed workers, but sometimes by contractors. So if a fence needs fixing, or a door, or a faulty heater, the council comes and fixes it.