As Sophie said, the hospital must respect your confidentiality. Your employer would have to ask for your records in writing and you would need to give your written consent. Employers have a right to make sure their employees aren't skiving and so they're entitled to check whether you've attended hospital as you said. However, they're only entitled to relevant information - in this case probably a letter or memo to say that you were there at a given time, and possibly the department/consultant details. To ask for details of your condition or any other personal information would be out of order and such information most likely refused.