1. identify what the policy is on plagiarism. It should be abailable as well as the procedure. that way you will know what their criteria is and how they apply it. the details should have been in the e-mail.
2. Agree with the people who have pointed you to the SU to get guidance. they will ahve dealt with this before and can signpost you.
3. What gavin said at #7. Find your essay plus drafts that are time stamped. this will help uoi show how you planned and wrote the essay. You need to identify the bits where you have missed quotation marks. You also need to check if you provided a url whether there was adequate. That will give you an idea of just how many uncredited segments there are. Normally you would footnote things, so I doubt lack of " marks will be terminal.
4. Now you are prepared just listen to what the lecturer has to say. Hopefully any area he has concerns about then you will be able to show it was an error. By the end of the meeting be sure to find out what the next stage is and when you will find out by.
5. If they take it forward and the consequences are serious, then go back to the SY and take their advice about representation.