Thanks for the reply. You are correct, users can sign up for the 2 day event and then the optional 3rd day separately if they wish. But for some reason it was saying users were already booked for the 28th of August even though they had only signed up for the 23 & 27th. 

I've had this set up this exact way for another seminar and it worked perfectly fine as 2 separate sign ups. I'm not sure why this one was linking like this. 

I've set up the 3rd optional day as a complete new event to stop it linking. 

Thanks again