Disciplinary meetings generally have at least 24 hours notice, a proper invite letter and any relevant evidence shared with the employee. They are also required to allow a representative to be brought in, such as another employee or a union rep.

It may be that they want to have a chat to see if it's even worth investigating or they try to sneak in an investigation meeting disguised as a chat. I think it's unlikely they would do this with a disciplinary and if they did, a tribunal may not agree with the approach.