Yeah, they are something you should always be pinned up. Think about it this way; for less than £50, you could get the kind of construction site signage you need. This gives you peace of mind that staff can easily see instructions and should help remove any worries about a legal threat if something goes wrong.
The other side of this is that your staff will be generally safer. They will see clearly marked signage, which means they do not walk into the wrong place. The cost of this signage will be multiples cheaper than even the cheapest lawsuit for negligence.
Buy your signage from a reputable source, and you can make sure that you have easily visible, clearly legible site barriers that stop people from making dangerous decisions at work.