I would guess that you have much more chance of being sued by going for the restricting access theme than you have of being prosecuted for having a fire while a 'difficult case' was on an upper floor. But there is a great difference between being sued under the DDA and someone coming to harm in your premises because you didn't take reasonable measures to ensure their safety.
I think we have to get out of the mindset of "getting sued" and look at the situation practically and sensibly.
I'll expand on my original answer.
Lets assume the premises provides a certain service to members of the public above ground floor level which isn't accessible to persons with mobility impairments. Clearly this is unacceptable.
The building is old and it would be difficult to modify the existing lift to an evacaution lift without substantial and disporportionate cost. It would also be too expensive to install a brand new lift.
Evac chairs can be used and may be suitable for some mobiltiy impaired people, but not all. To buy all manner of different evacuation aids simply on the off chance one of them will suit an individual would be costly and unreasonable. Buying aids which would suit the vast majority of impaired persons isn't unreasonable.
So what do you do about it?
Firstly can the services being provided on the upper floors be located to ground floor level? If that is not practicable can those services be replicated on a smaller scale at ground floor level?.
Could it be that a mobility impaired persons presses a bell / phones and someone comes to their attention on the ground floor? (This is what happenes in a lot of banks)
There are always solutions which are "reasonably practicable" to implement.
On the flip side if you employ a less abled person(s), then you will know much more about those individuals circumstances and thus be able to tailor an evacuation plan to suit (PEEP) and if needs be buy the right evacuation aid to assit them.
One thing you must not rely on is fire crews evacuating people for you. The brigade's function is to undertake rescue, and then to fight fire. They can not effectively do this if their resources are tied up evacuating people, particularly in areas covered by the RDS fire crews where resources will already be tight, and back up appliances may take time to arrive.
As someone already pointed out you need to balance access with safety, the consequences of being prosecuted by the fire authority are more severe potentially than getting sued
I slightly disagree with Davidmc's comments. They may hold true to an extent, but there is a myth that exists that an organisation will be sued to the hilt if they dont provide x,y,z .
Keep in mind the statement "reasonably practicable". The law requires you to do everything "reasonably practical" to allow access to less abled persons. So long as you have demonstrated that you have done everything reasonably practical then you can not be left open to legal action. But that does not mean you have to spend millions of pounds upgrading a builidng, nor indeed put people at risk simply because a solution can't be found.
