It depends what is in your organisation's disciplinary and grievance policy, but yes in my employer. Is this meeting investigating some alleged misdemeanour or poor practice?
When I worked in the NHS, not for an investigatory meeting but yes if there had been a complaint against you, or you were being investigated for a breach of the rules.
So First thing a letter
then investigatory meetings could be arranged verbally
then a letter summoning you to disciplinary meeting.
Of course if the investigatory meeting is regarding someone elses misdeed then no letter necessary.
The short answer is no
If your employer has a disciplinary code then he has to follow it and that see above may specify the steps.
if he doesnt have a code and there is no reqt to have one ( but there is a reqt that if he has one he must follow it ), then he has to follow the rules of natural justice ( The ACAS site has a lot about this in employment law ) and there is no rule of nat justice that they have to give you a letter.
Make sure you join a union and take him ( the rep - him or her) along
or else take an other work colleague - ACAS states the results are muich better for the workers if you do this.