I worked for a supermarket for over 8 years, in that time I was disciplined and suspended a few times, one thing you need to do is join the union asap, even if it wont help you for this case, supermarket managers tend to attempt to do as they please, getting away with 'it' because no one else knows their rights and the managers don't really know how properly to behave either.
The process I was always led down, was, the first initial meeting where they discuss with you the issue and whether or not its going forward.
Then they inform you are being suspended, then you'll get the letter inviting you in for a disciplinary meeting. In every meeting you are entitled to union representation or a witness.
I've no idea what you've done, have you ever grazed or something along those lines? could be they've been looking back through cctv and investigated you for something over the last few weeks?
Depending on the severity or whether they like you, chances are you'll get off with a written warning or final written, either way you'll be entitled to appeal and should regardless of outcome, even if its just to reduce the punishment.
I believe because you were not informed first of the issue in the initial meeting they will not be able to uphold anything as they haven't followed the correct procedure. They have to follow it by the book for anything to stick.