"unsealed Letter"

Today 14th May I received my appointment letter to attend the eye clinic at Bolton Royal 19th August 2019, the letter was sent to me in an unsealed envelope the flap had not been glued down which I would have said is totally out of order, on the letter is my NHS No, and the hospital Ref No, anyone could have taken the letter from the envelope and read the contents so where's the privacy, I feel that your staff need further training or to be more careful with outgoing mail