External fire exit is a vertical ladder from the upper plant room to ground level.  I have a concern that there is a chain at the top of the ladder but not at the bottom.  We recently have had some climb the ladder, how can we restrict access to it without blocking the ladder

A fire exit by vertical ladder? Someone climbing the ladder (presumably not an employee).

Most common ladder barriers work on the basis of padlocking a plate over the lower rungs to prevent access but then how does someone climbing down safely remove the padlock (typically located for someone stood at ground level)?

You could fence in the foot of the ladder but then how to control what would presumably be a gated exit?

As it is solely for emergency what about a ladder whose lower portion is kept in a raised position until a pin or similar is released? You then have to ensure adequate maintenance of the mechanism.

So tempting to suggest a firemans pole in replacement but then if they climb ladders what stops them shimming up a pole and how does someone with an injury use a pole in an emergency?

Eliminate the climbers access to the area? Change the escape to something more suitable?

You say this is a Plant Room and I assume the employees who access this room are not disabled in any way.  It does seem to disallow anyone with disabilities from working there.  Is there a plan going forward to modify the fire exit.

My opinion about ladders used as means of escape in not recommended.