I have just made an appointment at my local family planning. Like the others have said the first appoinment is for questions, suitability and a smear.
If all is good from there then a 2nd appoinment is required for insertion.

I havent had the Mirena but I have had a normal IUD a few years ago and although a bit uncomfortable afterwards (like bad period cramps) I was good to go the next day.
I did have extremely heavy periods with the IUD thoguh so im hoping that the Mirena will knock that out.