You must do what is right for your business.
Keep accurate records of every reasonable attempt to rectify the situation (and you must make reasonable attempts prior to any dismissal, such as re-training or re-distributing the job tasks etc.)
You only need to show that your business is suffering financially and progressively because a member of staff is unable to carry out work essential to its operations, to warrant dismissal. If no other position is available to the employee, you have no legal obligation to create one.
Consider acts such as the DDA as you would for any other 'employee rights' issue, but don't let it frighten you into NOT taking action that will be to the detriment of your business.