Any idea whether we need to select the flat immediately at the stated time of appointment or can we wait awhile to slowly choose?
On your appointment day, you will need to go in and select your unit immediately. It like buying movie ticket over the counter.
Try not to select too long as the one behind you will have to wait till you done.
Note: If you are late for your appointment on that day, you will be placed at the end of the queue for that day. So be there early on the appointment date.