Getting someone to sign something doesn't help at all.
It is the responsibility of the company to ensure that through risk assessment all and any risks have been identified and removed/reduced (through removing/re-working the task, method statements, safe systems of work, issue of PPE, training etc). You cant sign away your own or an employees safety.
You can have as many signed bits of paper as you want but without the previous you would be screwed. If the RA is effective and the resultant risks are manage correctly as a company you should not be liable. Wouldn't stop a shyster no win no fee lawyer trying to build a case mind.